Klebsiella pneumoniae : Prevalence,Reservoirs,Antimicrobial Resistance,Pathogenicity,and Infection: A Hitherto Unrecognized Zoonotic Bacterium

Abstract

Klebsiella pneumoniae is considered an opportunistic pathogen, constituting an ongoing health concern for immunocompromised patients, the elderly, and neonates. Reports on the isolation of K. pneumoniae from other sources are increasing, many of which express multidrug-resistant (MDR) phenotypes. Three phylogroups were identified based on nucleotide differences. Niche environments, including plants, animals, and humans appear to be colonized by different phylogroups, among which KpI (K. pneumoniae) is commonly associated with human infection. Infections with K. pneumoniae can be transmitted through contaminated food or water and can be associated with community-acquired infections or between persons and animals involved in hospital-acquired infections. Increasing reports are describing detections along the food chain, suggesting the possibility exists that this could be a hitherto unexplored reservoir for this opportunistic bacterial pathogen. Expression of MDR phenotypes elaborated by these bacteria is due to the nature of various plasmids carrying antimicrobial resistance (AMR)-encoding genes, and is a challenge to animal, environmental, and human health alike. Raman spectroscopy has the potential to provide for the rapid identification and screening of antimicrobial susceptibility of Klebsiella isolates. Moreover, hypervirulent isolates linked with extraintestinal infections express phenotypes that may support their niche adaptation. In this review, the prevalence, reservoirs, AMR, Raman spectroscopy detection, and pathogenicity of K. pneumoniae are summarized and various extraintestinal infection pathways are further narrated to extend our understanding of its adaptation and survival ability in reservoirs, and associated disease risks.

Introduction

K lebsiella pneumoniae is a nonmotile, lactose-fermenting bacillus belonging to the Enterobacteriaceae family. It is a member of the ESKAPE group of bacteria (which includes Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species), often expressing a multidrug-resistant (MDR) phenotype (Karaiskos and Giamarellou, 2014; El-Mahallawy et al., 2016). The presence of a thick polysaccharide capsule distinguishes this pathogen from majority of Enterobacteriaceae, a feature that complicates its detection by the host's immune system, thereby failing to be engulfed by phagocytes (Kabha et al., 1995).

Interactions between humans, animals, and the environment can have an impact on public health, as a source of diseases. According to the World Health Organization (WHO) definition, diseases, naturally transmissible from animals to humans through direct contact or food, water, and the environment, are commonly referred to as zoonoses. At least 61% of all human pathogens are zoonotic, and zoonoses comprise a large percentage (75%) of all newly identified or emerging infectious diseases as well as existing infectious diseases during the past decade. With the exception of several newly emerging zoonoses such as SARS and influenza H5N1, most are not prioritized by health systems at national and international levels, and are therefore labeled as neglected. Although not listed in the WHO zoonoses inventory, K. pneumoniae was reported as a zoonotic disease in several publications (Whitehouse et al., 2010; Soto et al., 2012; Zhang et al., 2019; Santaniello et al., 2020).

Taxonomy and Phylogroups of K. pneumoniae

Based on the sequence of several genes and whole-genome sequencing data of more than 300 human and animal isolates from four continents, K. pneumoniae can be divided into three distinct phylogroups: these comprise K. pneumoniae (KpI) (Fevre et al., 2005), K. quasipneumoniae (KpII A-B) (Brisse et al., 2014), and K. variicola (KPIII) (Rosenblueth et al., 2004; Maatallah et al., 2014), although all are able to colonize and infect humans, KpI is frequently associated with human infection (Holt et al., 2015).

Among these three phylogroups, a pangenome of 29,886 unique protein-coding sequences can be detected. Furthermore, among 328 K. pneumoniae genomes studied, these shared ∼1900 core genes, giving a median of 5705 genes per genome. The remaining genes are considered to be accessory and more than 65% of these are considered rare. Some of these accessory genes are likely to be localized on plasmids acquired from a wide range of bacteria, including other members of Enterobacteriaceae, Vibrio species, and Acinetobacter species (Holt et al., 2015). Genomic plasticity along with its ability to exchange plasmids with other species makes this bacterium an important reservoir of antimicrobial resistance (AMR) and virulence genes (Ramirez et al., 2014).

To gain a better understanding of the biology of this clinically important bacterium, further large-scale comparative and functional genomics studies are required (Wyres and Holt, 2016).

Based on a recent review, the systematics of K. pneumoniae was updated by performing population genomics, and a whole genome-based tree was generated from representative complete genome sequences to show the phylogenetic relationships between the K. pneumoniae species complex (KpSC), other select members of the Klebsiella genus, and family Enterobacteriaceae, and to advance our understanding of K. pneumoniae taxonomy, evolution, diversity, and distribution (Wyres et al., 2020). From this phylogenetic tree, five species and/or seven phylogroups were included in the K. pneumoniae complex (“Kp”) with KpII and KpIII being split into two subspecies: K. pneumoniae sensu stricto (Kp1), K. quasipneumoniae subsp. quasipneumoniae (Kp2) (Brisse et al., 2014), K. quasipneumoniae subsp. similipneumoniae (Kp4) (Brisse et al., 2014), K. variicola subsp. variicola (Kp3) (Rosenblueth et al., 2004), K. variicola subsp. tropica (Kp5) (Rodrigues et al., 2019), K. quasivariicola (Kp6) (Long et al., 2017), and K. africana (Kp7) (Rodrigues et al., 2019).The members in KpSC share only 90% average nucleotide identity with other Klebsiella species and K. pneumoniae sensu stricto is highly prevalent in clinical collections, typically comprising ∼85% of the isolates identified as K. pneumoniae; hence, the “Kp” abbreviation usually referred collectively to the seven phylogroups of the KpSC and “K. pneumoniae” was reserved for K. pneumoniae sensu stricto (phylogroup Kp1) (Barbier et al., 2020; Wyres et al., 2020). All species have been isolated from the human gut, and all, but Kp5, have been isolated from human infections (Rodrigues et al., 2019; Wyres et al., 2020).

Epidemiology of Klebsiella

Human diseases caused by Klebsiella

Despite being benign in many instances, this bacterium is often linked with community-acquired (CA) or hospital-acquired (HA) infections. HA K. pneumoniae are responsible for bloodstream infections, along with infections of the abdominal cavity, surgical sites, soft tissues, meningitis, and respiratory and urinary tracts, particularly in immunocompromised patients, the elderly and neonates (Podschun and Ullmann, 1998; Lee and Burgess, 2012; Janda, 2015). The most common site of infection is the blood (52%), followed by the respiratory tract (30%), and urine (10%) (Lee et al., 2012). However, since the 1980s, a new more virulent variant of Klebsiella has been disseminating globally, being responsible for CA K. pneumoniae. This strain was first described in the Asian Pacific Rim and has now spread globally (Pastagia et al., 2008; Sobirk et al., 2010; Decré et al., 2011; Fierer et al., 2011; Vila et al., 2011). This variant is known for its hypervirulent (hypermucoviscous) phenotype and for the presence of specific capsule serotypes K1 and K2, along with various siderophores (Shon et al., 2013). This bacterium is capable of causing pyogenic liver abscesses (PLAs), Lemierre's syndrome, and antibiotic-associated hemorrhagic colitis, and has a propensity to migrate to other parts of the body with mortality rates ranging from 5% up to 30% (Janda, 2015).

Molecular methods for subtyping Klebsiella isolates

There are several molecular methods that can be used for subtyping K. pneumoniae. These include repetitive sequence-based PCR, pulsed-field gel electrophoresis, multilocus sequence typing (MLST), and whole-genome sequencing. MLST is the most commonly deployed method to identify K. pneumoniae and is based on genetic variation detected among seven housekeeping genes (rpoB, gapA, mdh, pgi, phoE, infB, and tonB). The sequence type (ST) is determined based on the different allelic numbers assigned to each distinct sequence allele within each locus (Diancourt et al., 2005). The ST numbers are then combined into closely related groups to form clonal complexes (CCs) (Turner et al., 2007). Recently a new approach proposed to group these CCs into clonal groups (CGs). In this study, the most predominant STs would be considered the center of the CG and include both single-locus variants and their respective variants (Breurec et al., 2013). This approach would allow the identification of epidemiologically related strains to provide more meaningful information regarding their clonal evolution to the clinicians.

Epidemic Klebsiella CGs

Epidemic K. pneumoniae are restricted to specific clonal lineages (Lai et al., 2019). HA infections are known to be caused by MDR strains, especially those that are resistant to carbapenem-type compounds, primarily by acquiring the carbapenemase-encoding genes on mobile genetic elements (MGEs). These isolates normally belong to the CG258, comprising the ST258 and ST512 types (Bowers et al., 2015; Kelly et al., 2019; Palmeiro et al., 2019; Lai et al., 2019). In contrast, CA infections are known for their extreme virulence and limited resistance to antimicrobial agents and are normally associated with the CG23 group (Passet and Brisse, 2015; Lam et al., 2018; Lai et al., 2019), along with other CG groups, including CG65 and CG86 (Brisse et al., 2009; Decré et al., 2011; Passet and Brisse., 2015). However, several exceptions have been noted and it has been suggested that the evolution of K. pneumoniae could be due to homologous recombination rather than the accumulation of mutations (Brisse et al., 2009). The presence of hypervirulent and antimicrobial-resistant K. pneumoniae in food matrices poses a potential hazard to public health (Zhang et al., 2018). Although K. pneumoniae populations carry both drug resistance and hypervirulence markers and are found to be largely nonoverlapping in BIGSdb-Kp, a database that could enable rapid extraction of medical and epidemiological information from K. pneumoniae genomes, isolates with these features are still being detected (Bialek-Davenet et al., 2014).

Source of infection

Klebsiella is considered an opportunistic pathogen that can be found in the environment (Seidler et al., 1975; Barati et al., 2016), and in the gastrointestinal (GI) tracts of humans and animals (Bagley, 1985; Kim et al., 2005). It has the ability to adapt to an oxygenated or nonoxygenated environment (Keynan and Rubinstein, 2007). It is also found on the skin, and it can be cultured from the nose and throat of healthy individuals as a benign colonizer (Imhoff, 2005; Brisse et al., 2006; Berrazeg et al., 2013). In terms of Klebsiella of food origin, the following section describes the prevalence and reservoirs in this context.

Prevalence and Reservoirs of Foodborne Klebsiella species

Klebsiella can be found in different environments depending on the phylogroup. K. pneumoniae (KpI) is commonly associated with mammalian infection and can be found in hospital- and community-based environments. MDR and virulent CGs, CG258 and CG23, belong to this phylogroup. K. quasipneumoniae (KpII) can also be found in the same settings as KpI; however, this phylogroup does not demonstrate the same virulence capacity as K. pneumoniae, acting more as a carrier than a pathogen (Holt et al., 2015), nonetheless the primary reservoir remains unknown. In contrast, KpIII, K. variicola, has a specific environmental niche. This bacterium is commonly associated with plant-based infections (Rosenblueth et al., 2004; Wei et al., 2014; Martínez-Romero et al., 2015; Zurfluh et al., 2015). Ultimately, all three pathogenic species can cause human infections.

It had been recognized for a long period that hospital inpatients are more likely to be intestinal carriers of Klebsiella when compared with the nonhospital population, a feature that may probably be related to antimicrobial therapy (Rose and Schreier, 1968). Furthermore, until recently, K. pneumoniae was described as a human pathogen with attention being focused on related nosocomial infections (Campos et al., 2016). Recently, foodborne outbreaks associated with the Klebsiella genus have been reported with increasing numbers in different countries (Zhang et al., 2018). Similarly, gastroenteritis associated with contaminated turkey occurred at a catered company meal (Rennie et al., 1990). K. pneumonia, from a contaminated hamburger, was reported officially as an enteroinvasive foodborne pathogen for the first time in 1998 (Sabota et al., 1998); in this case, a previously healthy male ingested a portion of a hamburger from a fast food chain, after which, symptoms of gastroenteritis quickly developed into multiorgan failure. This report highlighted a possible hitherto unrecognized environmental niche for Klebsiella and that had not previously been reported as enteroinvasive.

The prevalence of Klebsiella species has been identified in various food products around the world and the food categories vary, consisting of turkey, pork, cake, cattle, chicken, milk, retail meat product, hamburger, fish and prawn, and even infant formula milk powder. It was reported as a frequent isolate among Enterobacteriaceae cultured from meats and meat-handling surfaces in packing plants. Occurrence reports of K. pneumonia in street food samples were also reported from different states in Malaysia (Haryani et al., 2007) and Benin (Moussé et al., 2016). The category details of Klebsiella recovered from foods are shown in Table 1.

Table 1.

Food Categories of Klebsiella Recovered

Food categories	Food	Publication Year	Regions	References
Meat	Turkey	1990		Rennie et al. (1990)
	Meats	1981	Canada	Stiles and Ng (1981)
	Turkey	1991		Hatheway and Farmer (1991)
	Turkey, beef, and chicken	2005	United States of America	Kim et al. (2005)
	Retail meat	2010	Pakistan	Hassan et al. (2010)
	Calf and chicken meat	2011	Turkey	Gundogan et al. (2011)
	Retail meat products	2015	United States of America	Davis et al. (2015)
	Raw chicken meat	2016	Turkey	Tekiner and Özpınar (2016)
	Retail chicken meat	2017	India	Upadhyay et al. (2017)
	Retail meat	2018	China	Wang et al. (2018)
	Meats and pastries	2018	Japan	Tominaga (2018)
	Retail meat	2018	China	Wang et al. (2018)
	Broiler carcasses	2019	Germany	Projahn et al. (2019)
	Raw Beef	2019	North-West South Africa	Montso et al. (2019)
	Retail chicken skin	2019	United States of America	Xu et al. (2019)
Hospital food	FOOD in hospitals, canteens, and schools	1971	United Kingdom	Shooter et al. (1971)
	Hospital prepared food for patients	1978	United Kingdom	Casewell and Phillips (1978)
	Hospital food (Salads and cold meat)	1980	United Kingdom	Mary et al. (1980)
	Hospital food	1986	Canada	Horváth et al. (1986)
	Milk made by hospital food blenders	1987	United Kingdom	Kiddy et al. (1987)
Food animals	Broiler	2012	Japan	Hiroi et al. (2012)
	Pig fecal swab	2016	Brazil	Cerdeira et al. (2016)
	Food animal (camel)	2019	Tunisia	Saidani et al. (2019)
	Food-producing animals (pig)	2019	China	Zhang et al. (2019)
	Food animals	2020	China	Lv et al. (2020)
	Raw and RTE retail food	2020	Singapore	Hartantyo et al. (2020)
Vegetables and fruits	Raw vegetables	2012	Malaysia	Puspanadan et al. (2012)
	RTE vegetables	2015	South Korea	Kim et al. (2015)
	Radish, tomato, cabbage, capsicum, banana, dates, mango, papaya, and watermelon	2016	Oman	Al-Kharousi et al. (2016)
	Retail fruits	2019	China	Yang et al. (2019)
Milk	Power infant formula	2011	China	Yan et al. (2011)
	Bovine milk	2016	India	Koovapra et al. (2016)
	Raw milk	2017	—	Badri et al. (2017)
	Raw milk	2017	Lebanon	Diab et al. (2017)
Aquatic products	Marine fish, freshwater fish, and freshwater prawns	1992	India	Singh and Kulshreshtha (1992)
	Shrimp imported from Thailand	2012	United States of America	Nawaz et al. (2012)
	Fresh raw seafood and chicken, frozen raw food, cooked food	2016	China	Tian et al., (2017)
Others or syntaxic food	Hamburger	1998	United States of America	Sabota et al. (1998)
	Cheese	2017	Egypt	Hammad et al. (2017)
	Food	1964	Germany	Horváth et al. (1964)
	Food	1965	Hungary	Horvath and Hanny (1965)
	Various food (sweets and snacks)	1988	India	Kaur et al. (1988)
	Cusumber, chilli, coriander leaves, ck berry, tomato, minced meat, chopped meat, and ice cream	2009	India	Shahid et al. (2009)
	Food	2015	India	Babu et al. (2015)
	Retail food	2018	China	Zhang et al. (2018)
	Raw food (poultry meat, mutton meat, fish, fruit, and vegetables)	2019	India	Ghafur et al. (2019)

RTE, ready-to-eat.

Apart from clinical origins, it appears that this pathogen can be recovered from various food sources, a feature overlooked due to the absence of surveillance. Moreover, a foodborne nosocomial outbreak due to extended-spectrum β-lactamase (ESBL)-producing K. pneumoniae in a hospital setting in Spain was reported, which prompted an epidemiological investigation of a possible foodborne transmission of KpI (Calbo et al., 2011). In this case, 156 patients were colonized or infected, more than one-third of hospital kitchen—screened surfaces or foodstuffs were colonized and 15% of food handlers were found to be fecal carriers.

The transmission of Klebsiella species has also been reported from cattle farms and its dissemination is mediated by biting midges and houseflies (Mohammed et al., 2016). The bacterium may also be found contaminating vegetables fertilized with untreated wastewater (Ibenyassine et al., 2007), and in samples from pig farms (García-Cobos et al., 2015), tap and pond water (Babu et al., 2015), natural surface water of the Baltic Sea (Podschun et al., 2001), milk samples from dairy farms (Amosun et al., 2012; Sudarwanto et al., 2015), and air samples taken from broiler houses (Bródka et al., 2012). K. pneumoniae and K. variicola can also cause bovine mastitis (Kanevsky-Mullarky et al., 2014; Osman et al., 2014; Podder et al., 2014). The same pathogens were also found in cow's milk as part of the microbial community in subclinical mastitis cases (Locatelli et al., 2010; Bhatt et al., 2012; Saishu et al., 2014; Timofte et al., 2014; Ali Hassan Ahmed Al-Shammary, 2017).

The poor characterization of the STs identified from most of these studies hampers the identification of the bacterial reservoirs and provides little insight into how these bacteria may be associated with cases of human infection. Nonetheless, others reported a close phylogenetic relationship between clinical isolates and those cultured from retail sources (Davis et al., 2015; Hartantyo et al., 2020), bovine mastitis (Holt et al., 2015), livestock (Davis and Price, 2016), and food-producing animals (Wang et al., 2018). Moreover, it was reported that meat sources were more prone to be contaminated with MDR Klebsiella species when compared with clinical isolates (Davis et al., 2015), a feature that could be due to the use of antimicrobial agents in food-animal production. The loss of the MDR phenotype in clinical isolates could be due to the adaptation of bacteria to the new environment, losing nonessential genes that could affect the metabolism of the bacterium.

Once these hypervirulent phylogenetic lineages have been identified in the environment (Osman et al., 2014; Overdevest et al., 2014; Davis et al., 2015; Holt et al., 2015), it facilitates our ability to predict how cases of community-hypervirulent Klebsiella infections will emerge over the coming years.

Klebsiella species can be detected in the environment, especially in animal and plant production settings, and these could be a hitherto unexplored reservoir. Intercountry passage of opportunistic Klebsiella as contaminants of fresh produce and meats could have been one of the reasons for the dissemination of this pathogen, especially given the fact that no food safety surveillance is legally mandated for this bacterium (Zurfluh et al., 2015; Al-Kharousi et al., 2016).

AMR Detected in Klebsiella species

Several members of the Enterobacteriaceae family have the capacity for carrying and transmitting a wide variety of resistance-encoding genes through horizontal gene transfer. One of the areas of most concern, in respect of human health, is the rapid dissemination of AMR among K. pneumonia. Some of the major resistance mechanisms for K. pneumoniae are summarized in Table 2. In K. pneumoniae, MDR evolution is largely driven through the acquisition of AMR genes on diverse mobilizable plasmids (Navon-Venezia et al., 2017), such as CG 258, which is implicated in global spread of the K. pneumoniae carbapenemases (KPCs) (Chen et al., 2014b). In 2017, the etiological agent responsible for a fatal outbreak was a carbapenem-resistant K. pneumoniae belonging to CG258, which had acquired ICEKp in the chromosome plus iuc and rmpA2-encoding genes on a virulence plasmid (Gu et al., 2018).

Table 2.

Antimicrobial Resistance Genes or Mutations in Klebsiella pneumoniae

Antimicrobial class		Resistance genes	Gene mutations
β-Lactamases (bla genes)	ESBLs	CTX-M, SHV, TEM, VEB, OXA	ompK
	Carbapenemases	KPC, GES, OXA, NDM, CARB
	Others	CphA, AmpC, CMY, DHA, FOX, MIR, VIM, SIM, IMP, SCO, PSE
Aminoglycosides		aac, aadAB, aph, armA, ant	—
Tetracyclines		tetAB	arcAB, oqxAB, acrAB, rpsJ
(Fluoro)Quinolones		qnrABCDS, qepA, aac	gyrA, parC
Folate pathway inhibitors		sul1, sul2, sul3, dfrAB
Phenicols		catAB, cml, floR
Polymyxins		mcr-1, mcr-3, mcr-8	pmrAB, mgrB

ESBL, extended-spectrum β-lactamase.

K. pneumoniae is known for its diversity of AMR elements and was described as a key trafficker of drug resistance genes from the environment to the clinical. This is mainly due to its ability to co-transfer AMR genes arising from its wider ecological distribution, its significantly varied DNA composition, its greater AMR gene diversity, and a higher plasmid burden when compared with other Gram-negative opportunists (Wyres and Holt, 2018). The diversity in gene content and context found in the Klebsiella genome is mainly due to plasmids, bacteriophages, insertion sequence, and integrative conjugative elements (ICEs) (Holt et al., 2015; Mathers et al., 2015).

During the 1980s and 2000s, K. pneumoniae was recognized for carrying plasmids that encode resistance mechanisms to counter aminoglycoside, fluoroquinolone, and ESBL classes of compound. The latter are mostly composed of TEM-, AmpC-, CTX-, OXA-, and SHV-type enzymes (Soilleux et al., 1996; Brisse et al., 2000; Li and Lim, 2000; Nadjar, 2000; Pagani et al., 2000; Schønheyder et al., 2000; Tzouvelekis et al., 2000; Mathers et al., 2015; Popescu et al., 2017). This repertoire of MDR then necessitates the use of carbapenams as the only useful chemotherapeutic option to treat ESBL-linked infections. However, since the 2000s, an unprecedented crisis involving carbapenemases spreading globally and including genotypes such as bla _NDM, bla _KPC, bla _IMP, bla _OXA, and bla _VIM has been observed and reported upon (Liu et al. 2009; Curiao et al. 2010; Cuzon et al. 2010; Pournaras et al. 2010; Prior et al. 2010; Lee and Burgess, 2012; Tzouvelekis et al. 2012; Chen et al. 2014b; Hudson et al. 2014; Jin et al., 2015; Mathers et al. 2015; Zhang et al., 2015; Zheng et al., 2016). Those plasmids associated with carbapenem resistance-encoding genes often carry other resistance- and virulence-encoding mechanisms (Huang et al. 2013; Hudson et al. 2014; Hennequin and Robin, 2016; Zheng et al., 2016). This situation has been further complicated with the identification of the transmissible colistin resistance genes mcr-1 (Liu et al., 2016), mcr-2 (Xavier et al., 2016), and mcr-3 (Yin et al., 2017), together with chromosomal mutations in mgrB, pmrA, pmrB, phoP, and phoQ (Choi et al., 2016). Increasing numbers of reports are describing the mcr-1 gene in K. pneumoniae of human origin (Gu et al., 2016; Leangapichart et al., 2016; Rolai et al., 2016; Stoesser et al., 2016; Newtonfoot et al., 2017; Quan et al., 2017; Tian et al., 2017; Mendes et al., 2018), environmental origin (Zhao et al., 2017), fruit origin (Yang et al., 2019), and (food producing) animal origin (Davis and Price, 2016; Tian et al., 2017; Wu et al., 2016; Argudín et al., 2017; Kieffer et al., 2017; Saidani et al., 2019). Furthermore, KPCs producing K. pneumoniae have already been detected and which co-expressed colistin resistance carrying mcr-1 (Li et al., 2016; Aires et al., 2017; Novović et al., 2017). This finding highlights the potential for dissemination of KPC-producing K. pneumoniae carrying an mcr-1 gene and this development may be associated with MDR international clones. Continued selection for the transmission of these genes could pose a threat to infection control strategies and future clinical treatments.

Polymyxins and tigecycline are regarded as major treatment options for carbapenem-resistant Enterobacteriaceae (CRE) infections (Sheu et al., 2019). However, the emergence and global dissemination of mobile colistin resistance has limited the efficacy of polymyxins (Liu et al., 2016), and tigecycline consequently is one of the few remaining drugs of choice in cases of severe CRE infection. Tigecycline is an important compound used in the treatment of CRE infection, especially carbapenem-resistant K. pneumonia (CRKP) (Li et al., 2015). However, continued clinical use of tigecycline led to a reduction in its efficacy, which in turn is linked to high mortality rates resulting from CRKP, the most common type of CRE (Navon-Venezia et al., 2017; Wang et al., 2018; Lv et al., 2020). More recently, a new plasmidborne RND efflux pump-type tigecycline resistance determinant was identified and found to be widespread among K. pneumoniae isolates from food-producing animal origin, offering the potential for onward transmission (Lv et al., 2020).

Most of the large plasmids identified in K. pneumoniae have been found to be associated with increased morbidity and mortality (Ramirez et al., 2014). It has been suggested that K. pneumoniae cultured from blood isolates carry a 200-kbp plasmid, which includes iron transport systems and the mucoid phenotype-encoding genes (Chen et al., 2004). Recently, two plasmid-driven epidemics in which the genes bla _KPC and bla _NDM were combined in the same bacterium have been reported (Pesesky et al., 2015; Quiles et al., 2015; Zhu et al., 2015), creating superbugs that are refractory to chemotherapeutic management.

It is thought that the acquisition of multiple plasmids from MDR isolates could be associated with limited fitness cost (Sheppard et al., 2016; Buckner et al., 2018; Wyres et al., 2019; Nang et al., 2018). Interestingly, data showed that MDR K. pneumoniae clones could more easily acquire virulence genes than hypervirulent clones could acquire resistance genes (Wyres et al., 2019), indicating a greater need for global genomic surveillance encompassing AMR information of K. pneumoniae.

Treatment of K. pneumoniae Infection

AMR in MDR K. pneumoniae is an important public health issue for patients in clinical setting, thereby limiting optimal treatment options (Bassetti et al., 2018). More treatment failures are recorded in those cases receiving monotherapy, when compared to others who received combination therapy (49% vs. 25%; p = 0.01) (Lee et al., 2012). In terms of the higher level of antimicrobial resistance in Klebsiella and the increasing incidence of CRKP, combination therapies using carbapenems, polymyxins, tetracyclines, and fluoroquinolones are recommended and widely used. There were no major treatment effect differences recorded between the three most common antibiotic class combinations: polymyxin plus carbapenem, polymyxin plus tigecycline, and polymyxin plus aminoglycoside (Lee et al., 2012; Petrosillo et al., 2013; Bassetti et al., 2018). Nevertheless, the best combination for each infection type remains to be confirmed and the continued use of carbapenems in combination therapy is likely to continue (Petrosillo et al., 2013).

In situ Identification of Foodborne K. pneumoniae and Rapid Detection of MDR Phenotypes by Raman Spectroscopy

Conventional approaches for testing drug resistance in bacteria usually requires the monitoring of bacterial growth during incubation with different antimicrobial compounds. Although the methodology is standardized, this is time-consuming, delaying the diagnosis and subsequent prescription of an alternative and more efficient drug if indicated. Whole-genome sequencing techniques and deep-level transcriptomics are powerful tools to identify, localize, and explore the expression of MDR genes in pathogens using high-throughput sequencing platforms. These developments now make it feasible to screen specific genotypes in a given bacterial isolate. Analysis of raw sequencing data, however, still requires a mastery of several appropriate software programs. Moreover, in the context of rapidly evolving bacterial populations, and for K. pneumoniae in particular, repeated exposure to a large range of antimicrobial compounds can trigger the emergence of new MDR phenotypes, for which the susceptibility to the available antimicrobial drugs remains unknown.

To choose the most efficient drug for treatment, screening techniques are needed to rapidly test the efficacy of a set of antimicrobial compounds against a bacterial isolate of interest, as quickly as possible. This test, if possible, would provide information on the drug susceptibility of the bacterium without prior characterization of the species, and on a limited number of cells to reduce the duration of the growth phase before the analysis. Raman spectroscopy could potentially fill this gap.

Several reviews highlighted the power of Raman spectroscopy for the identification of bacteria (Pahlow et al. 2015; Stöckel et al. 2016; Lorenz et al. 2017), which is a label-free, nondestructive vibrational spectroscopy technique that can be combined with confocal microscopy for imaging biological samples. The Raman spectrum of a single live bacterium can be acquired within a few minutes with limited sample preparation. This analysis unveils the molecular fingerprint of the bacterium, providing access to broad information content describing the chemical composition and the structure of the biomolecules (DNA/RNA, proteins, lipids, and carbohydrates) within the sample. The spectral signature of one bacterium is the result of the unique combination of molecular vibrations of these biomolecules and can be used to identify groups and subgroups of bacteria (Stöckel et al. 2016). Liu et al. (2009) showed that K. pneumoniae exhibits a different Raman spectrum compared to Enterococcus feacalis, Listeria monocytogenes, Staphylococcus aureus, Mycobacterium tuberculosis, Mycobacterium gordonae, Escherichia coli, and Serratia marcescens. The identification and subtyping of bacteria in complex matrices or in line during food processing remain challenging, Raman spectroscopy represents a powerful tool for the detection of foodborne pathogens in situ (Witkowska et al. 2017; Yaseen et al. 2017; Zhao et al. 2018).

Raman spectroscopy has indeed become an easy-to-use, high-speed analysis technique for the identification of pathogens with applications in medicine, and more laterally in food safety (Yaseen et al. 2017). The main advantages are (i) instantaneous analysis, (ii) limited sample preparation, (iii) in situ analysis in complex matrices, and (iv) real-time analysis of live bacteria.

When combined with powerful multivariate statistical and discriminant analysis, Raman spectroscopy also allows discrimination of pathogenic and nonpathogenic bacterial subtypes (Hamasha et al. 2013). A Raman study list of β-lactamases has been summarized, which included SHV1/2/5, OXA1/24 β-lactamases, and carbapenemases, among others (Carey et al. 2015). Clones expressing different resistance phenotypes against antimicrobial compounds (cephalosporins and quinolones) could also be successfully identified among K. pneumoniae (Guyot et al. 2012; Cheong et al. 2017). Using the highly specific spectral signature of each bacterium, Raman imaging could further be used to screen for MDR bacteria embedded in a food matrix. This approach has great potential in food safety and quality control, opening the way to the on-line detection and identification of largely overlooked foodborne bacteria, provided that robust data analysis routines are implemented.

A variant of Raman spectroscopy called Surface-Enhanced Raman Scattering (or SERS) has proven very efficient in probing the cell wall changes of bacteria due to its capacity to locally enhance the Raman signal of the bacterial cell wall components. Therefore, discrimination between Gram-positive and Gram-negative bacteria is reproducible and any disruption of the cell wall or cell membrane can be assessed by real-time analysis of the live bacteria after incubation with different antimicrobial compounds (Prucek et al. 2012; Liu et al. 2009; Hlaing et al. 2018). Results have also been obtained by conventional Raman spectroscopy (Xuan Nguyen et al. 2017). As several antimicrobial compounds specifically target the bacterial cell membrane, this approach is promising for the rapid screening of the susceptibility of a bacterial isolate to a large set of antimicrobial drugs in a clinical or food safety context (Cheong et al. 2017). The mechanistic investigation of the effect of antimicrobial drugs by Raman spectroscopy could benefit in the future from the development of other techniques based on the Raman effect, such as SERS, Coherent Anti-Stokes Raman Scattering, and Stimulated Raman Scattering (SRS) (Evans and Xie, 2008; Hong et al. 2018).

When transmission occurs throughout a hospital, there is an urgent need for the use of a rapid subtyping strategy designed to detect and subsequently prevent further pathogen spread. Voor in't Holt et al. (2015) reported a clonal outbreak of ESBL-producing Klebsiella in a hospital. They concluded that subtyping should be continuously performed not only in the case of outbreaks but also in an apparent non-outbreak situation. Raman spectroscopy could be the rapid typing method easy to implement as an early identification and an immediate infection-prevention strategy to prevent further transmission of those small outbreaks identified early on.

Virulence and Pathogenicity in K. pneumoniae

As mentioned above, the GI tract is an important site for colonization of Klebsiella species (Fung et al., 2012). It is predicted that the majority of Klebsiella infections follow a similar pattern in humans as demonstrated by other food-poisoning pathogens, such as E. coli, L. monocytogenes, Salmonella species, or Campylobacter species (Strauman et al., 2010; Robinson et al., 2012; Gessain et al., 2015; Kirk et al., 2016).

K. pneumoniae employs various virulence factors (VFs) and strategies to survive in the host infection process. Paczosa and Mecsas (2016) reviewed four major classes of VF, which were well studied and described in K. pneumoniae, including capsule, lipopolysaccharide (LPS), siderophores, and fimbriae. Liu et al. (2019) separated and resolved the VFs of K.pneumoniae into several further detailed groups, especially for fimbriae and siderophores, with supplementary VF types in the virulence factor database (www.mgc.ac.cn/VFs/), and these VFs and their relevant genes were summarized in Table 3.

Table 3.

Virulence Factors and Related Virulent Genes in Klebsiella pneumoniae (Modified and Updated Based on Virulence Factors in Virulence Factor Database)

Virulence factor classes (subgroups)	Virulent genes	Detailed description	References
Adherence (Type I fimbriae)	fimA; fimB; fimC; fimD; fimE; fimF; fimG; fimH; fimI; fimK	Adhering to human mucosal or epithelial surfaces	Paczosa and Mecsas (2016)
Biofilm formation (Type 3 fimbriae)	mrkA; mrkB; mrkC; mrkD; mrkF; mrkH; mrkI; mrkJ	Mediating the formation of biofilms on biotic and abiotic surfaces	Paczosa and Mecsas (2016)
Efflux pump (AcrAB)	acrA; acrB	Mediate resistance against host-derived antimicrobial peptides	Padilla et al. (2010)
Immune evasion (Capsule)	cpsACP; galF; gmd; gnd; magA/wzy_K1; manB; manC; ugd; wcaG; wcaH; wcaI; wcaJ; wclY; wcsS; wcsT; wza; wzb; wzc; wzi; wzx	Assisting in evading the host immune system by protecting bacteria from opsonophagocytosis and serum killing	Paczosa and Mecsas (2016)
Iron uptake (Aerobactin)	iucA; iucB; iucC; iucD; iutA	Citrate-hydroxamate siderophore aerobactin is needed for successful infection by hypervirulent K. pneumoniae in pneumonic	Russo et al., (2014); Russo et al. (2015)
Iron uptake (Enterobactin)	entA; entB; entC; entD; entE; entF; fepA; fepB; fepC; fepD; fepG; fes; ybdA	Necessary for penetration to deeper tissues	Bachman et al. (2012)
Iron uptake (Salmochelin)	iroB; iroC; iroD; iroE; iroN	Involved in the acquisition of iron by K. pneumoniae, kfu was necessary for the formation of liver and brain abscesses. A major trigger of inflammation and bacterial dissemination induced during K. pneumoniae lung infection	Holden et al. (2016)
Iron uptake (Yersiniabactin)	fyuA; irp1; irp2; ybtA; ybtE; ybtP; ybtQ; ybtS; ybtT; ybtU; ybtX	The phenolate siderophore Ybt contributes to evasion of the activity of lipocalin2 in the lung—a host factor that neutralizes enterobactin-based iron acquisition	Holden et al. (2016)
Iron uptake (Kfu)	Kfu	An ABC iron transport system that is involved in the acquisition of iron by K. pneumoniae, and there is a strong association between the expression of this factor and HV K. pneumoniae strains	Fang et al. (2007); Paczosa and Mecsas (2016)
Nutritional factor (Allantion utilization)	allA; allB; allC; allD; allR; allS	Providing a nitrogen source to increase virulence in K. pneumoniae at certain sites of infection	Chou et al. (2004)
Regulation (regulation of capsule synthesis)	rcsA; rcsB	RcsB combined with the unstable auxiliary regulator RcsA to bind to an RcsAB box in the promoter region to upregulate the cps genes	Su et al. (2018)
Regulation (regulator of mucoid phenotype A)	rmpA; rmpA2	Positively regulate the cps locus at the transcriptional level, resulting in the hypercapsule phenotype	Hsu et al. (2011)
Secretion system (T6SS)	clpV/tssH; dotU/tssL; hcp/tssD; icmF/tssM; impA/tssA; sciN/tssJ; tle1; tli1; tli1; tli1; tli1; tssF; tssG; vasE/tssK; vgrG/tssI; vipA/tssB; vipB/tssC	Antibacterial activity	Liu et al. (2017)
Serum resistance (LPS)	glf; kfoC; wbbM; wbbN; wbbO; wzm; wzt; uge; wabG	LPS production. Resistant to serum complement. Also, play a role in protecting bacteria from antimicrobial peptides, including polymyxin antibiotics	Paczosa and Mecsas (2016)
Toxin (Colibactin)	clbA; clbB; clbC; clbD; clbE; clbF; clbG; clbH; clbI; clbJ; clbK; clbL; clbM; clbN; clbO; clbP; clbQ; clbS	Inducing DNA damage	Lu et al. (2017)

LPS, lipopolysaccharide.

There are several ways by which Klebsiella species can be transmitted to susceptible individuals; infections can be transmitted through contaminated food or water, particularly in CA-type scenarios (Davis and Price, 2016). The bacterium can also be transmitted through animal-to-person contact, person-to-person contact, or air droplets (Shon et al., 2013). The latter two routes are the most common means involved in HA cases of infection (Ridolfo et al., 2016).

Extraintestinal infections are a common event caused by Klebsiella and, in particular, linked with hypervirulent isolates. These bacteria are able to ascend into the bladder from the perineum (Dedeić-Ljubović and Hukić, 2009), or translocate the GI tract into the peritoneal cavity or other extraintestinal locations to cause PLA among other metastatic infections, such as meningitis and endophthalmitis (Liu et al. 2013; Hsu et al., 2015; Kazanji et al., 2016). It is reported that translocation through the GI tract could be the most likely route leading to K. pneumoniae causing PLA (Chung et al., 2011; Fung et al. 2012). Hypervirulent K. pneumoniae (hvKp) is now becoming increasingly recognized as a pathogen of concern. Some of the distinguishing phenotypes may include emergence of XDR strains, an increase in capsule, and siderophore production by hvKp relative to classical K. pneumoniae (Sellick and Russo, 2018). However, specific studies describing the epidemiology and transmission of hvKp are lacking.

These following lines will focus on the manner in which Klebsiella causes extraintestinal infections such as PLA, or bacteraemia through the oral route by crossing the intestinal mucosa layer.

Infection Pathway Traversed by Klebsiella species

When food contaminated with Klebsiella is ingested, the pathogen will transit through the GI tract to colonize the intestine (Fig. 1A). Along this pathway, Klebsiella will be exposed to several hydrolytic enzymes, variations in pH, and competition with other bacteria together with the host's immune system. K. pneumoniae, like other pathogens, is susceptible to the action of phospholipases, amidases, and especially lysozyme, a common enzyme present in the mucosal surfaces that degrades peptidoglycan (Markart et al., 2004). The presence of stomach and bile acids presents an additional barrier to bacterial survival. Bile acids are released into the duodenum and are found along the GI tract (Anes et al., 2015). The goblet cells of the epithelial barrier are responsible for the production of highly glycosylated mucins and other mucin-derived products that are the most abundant proteins in the intestinal lumen (Fig. 1B).

FIG. 1.

(A) Schematic showing Klebsiella pneumoniae and the pathway it takes along the gastrointestinal tract following ingestion of contaminated food. (B) Host's innate immune mechanisms triggered to fight bacteria in the gut and representation of the pathway used by K. pneumoniae to colonize and translocate across the intestinal epithelial barrier. (C) Elucidation of the mechanism used by Klebsiella to evade macrophage phagocytosis. Color images are available online.

Antimicrobial proteins (AMPs), such as defensins, cathelicidins, and lysozyme, are released into the lumen mucus by Paneth cells. Together with the presence of immunoglobulin A, they constitute an initial line of defence against pathogens (Peterson and Artis, 2014). Gram-negative bacteria also possess intrinsic efflux mechanisms such as AcrAB-tolC, which aid their survival in harsh environments (Anes et al., 2015). In Klebsiella species, Klebsiella-specific efflux pumps such as EefXABC (Coudeyras et al., 2008), which confers resistance to inorganic acids and hyperosmolarity (the same acidic conditions that can be found in the stomach), or the KpnEF (Srinivasan and Rajamohan, 2013) and KpnGH (Srinivasan et al., 2014) pumps are also active and these can help the pathogen survive in the presence of AMPs and bile stress, giving the bacterium time to adapt to the environment. To overcome these host defence mechanisms, particularly the enzymatic-mediated strategies, Klebsiella undergoes several modifications of its capsule, peptidoglycan, and LPS so the hydrolytic activity of these enzymes is compromised (Moranta et al., 2010; Hsieh et al., 2013; Bachman et al., 2015; Llobet et al., 2015). It has also been reported that the genes pgaABCD, which are involved in the synthesis and secretion of poly-β-linked N-acetylglucosamine, mediate biofilm formation under bile salt stress (Chen et al. 2014a). The stress induced by all these defence mechanisms works also as a signaling trigger to prepare the bacteria for the next step, the colonization of the epithelial cells. Urea is found in the gut in abundance; the enzymatic ability to hydrolyze urea to ammonia is due to the presence of the ureDABCEFG operon, which contributes to the active growth of K. pneumoniae, improving its ability to colonize the host's GI tract (Maroncle et al., 2006). Another metabolic advantage expressed by Klebsiella is the ability to metabolize allantoin to carbon and nitrogen. The allantoin operon has been found to be associated with hypervirulent strains causing liver abscess (Yu et al., 1991; Chou et al., 2004).

Adhesins and biofilm formation in K. pneumoniae

To colonize the intestine, Klebsiella requires the necessary machinery to adhere to epithelial cells. Adhesins play an important role in initiating the process of colonization; however, not all are involved in adhering to the host's cells directly. There are six types of fimbriae in Klebsiella responsible for this process: type 1 fimbriae, type 3 fimbriae, Kpc fimbriae, E. coli common pilus (ECP), KPF-28 adhesin, and the nonfimbrial antigen CF-29K.

Type 1 fimbriae are encoded by the locus fimBEAICDFGHK and are common in all Enterobacteriaceae. These structures are thin, thread-like surface protrusions located on the outer membrane, composed of repeating FimA subunits with a FimH molecule at the base and whose expression is regulated by fimK (Struve et al., 2009). Other regulators such as bamB appear to be also involved (Hsieh et al., 2016). The remaining genes within this cluster, fimF and fimG, code for small structural subunits that confer some variability to these fimbriae. The genes fimB and fimE encode two recombinases that are responsible for the switch between the fimbriated and nonfimbriated states. Similarly, fimC encodes for a fimbrial chaperone and fimD is expressed as an usher protein (Struve et al., 2008; Paczosa and Mecsas, 2016). These fimbriae bind to D-mannosylated glycoproteins present in the host or extracellular matrix surfaces (Stahlhut et al., 2009).

Type 3 fimbriae are encoded by the mrkABCDFJIH gene cluster and it is present in almost all K. penumoniae (Langstraat et al., 2001). In a similar arrangement as for the structure of type 1 fimbriae, type 3 fimbriae consist of several MrkA subunits protruding from the outer membrane in a helix-like structure, with the adhesion MrkD located at the tip. MrkB is a chaperone and mrkC encodes an usher protein (Struve et al., 2009), while MrkF is involved in the surface stability of the fimbriae. The remaining genes mrkJIH are involved in regulation and expression under different conditions (Johnson et al., 2011; Tan et al., 2015). Type 3 fimbriae are mannose resistant in contrast to type 1 fimbriae. These structures mediate attachment in vitro to human endothelial and urinary bladder cell lines (Tarkkanen et al., 1997), and in coated matrices with proteins such as type IV and type V collagen (Tarkkanen et al., 1990; Jagnow and Clegg, 2003).

Notably, type 1 and type 3 fimbriae have no effect on the ability of K. pneumoniae to colonize the intestine or the lung. Type 1 fimbrial genes were found to be upregulated in urinary tract infections (Struve et al., 2008, 2009), a similar observation also being made for type 3 fimbriae that have the ability to colonize the bladder (Tarkkanen et al., 1997; Murphy et al., 2013). Fimbriae of type 3 also seem to play an important role in biofilm formation (Schroll et al., 2010). Biofilm formation is an important step in protecting bacteria from the epithelial antimicrobial agents, conferring an improvement in the ability of Klebsiella to survive in the intestine. The ability of K. pneumoniae to bind to surfaces such as indwelling catheters or other medical devices provides it with an ability to seed vulnerable sites and to persist in patients (Paczosa and Mecsas, 2016). Deletions of both fimbriae type 1 and type 3 do not appear to attenuate virulence in a lung infection model (Struve et al., 2009), indicating that other factors and/or fimbriae play a role in infection.

Another fimbria that confers strong biofilm forming activity is the Kpc fimbriae. This type of fimbriae is encoded by the kpcISABCDEFG operon and act in a similar manner as the type 1 and type 3 fimbria systems (Wu et al., 2010). Interestingly, this system was not found to be expressed under any in vitro condition studied, leading to speculation about a specific colonization condition or environmental stress. Kpc fimbriae were found to be most prevalent in clinical isolates expressing hypervirulent phenotypes (Wu et al., 2010).

The ECP homolog, present in the K. pneumoniae genome, is characterized by the operon ecpABCDE and has been shown to be involved in cell adherence, maintaining a strong physical contact between the host and the bacteria (Alcántar-Curiel et al., 2013). Currently, little is known about this pili system in K. pneumoniae.

KPF-28 adhesin is a long, thin flexible fimbria located on a transmissible R plasmid that also encodes the CAZ-5/SHV-4 β-lactamase enzyme, and the former facilitates adherence to Caco-2 cells in vitro. The first time these fimbriae were reported was in Claremont-Ferrand, France, in clinical urinary tract infection (UTI) cases (Di Martino et al., 1996). Another example of a nonfimbrial adhesin capable of adhering to Caco-2 cells is the CF-29K. The gene encoding this 29-kDa protein was located on a conjugative plasmid in K. pneumoniae (Darfeuille-Michaud et al., 1992; Di Martino et al., 1995). Little is known about the manner by which these fimbriae and non-fimbriae function to attach the bacterium to intestinal cells. It has been reported that the Klebsiella capsule can impede cell adherence and invasion, whereas LPS contributed to adhesion in a nonspecific manner (Sahly et al., 2000; Clements et al., 2008).

Although the correlation between fimbrial expression and the attachment of the pathogen to the host's intestine is not fully elucidated, it is clear that the formation of biofilm renders K. pneumoniae more resistant to the host defences and improves its capability to colonize epithelial cells. The formation of a biofilm, in addition to the presence of fimbriae described above, involves metabolic and structural changes in the capsule and LPS (Balestrino et al., 2008). K. pneumoniae biofilms are mainly composed of mannose (40–50%) (Bales et al., 2013). In vitro studies indicate that genes involved in amino acid synthesis and in l-arabinose metabolism (Stahlhut et al., 2010), as well as sugar phosphotransferases (Boddicker et al., 2006) play an important role in biofilm formation. Similarly, the deletion of the gene involved in the transport of cellobiose (celB) attenuated the ability of Klebsiella species to form biofilms and to colonize the intestine (Wu et al., 2012). Moreover, deletion of the gene treC that encodes for a trehalose-6-phosphate hydrolase and the deletion of the gene sugE resulted in decrease of biofilm formation as a result of capsule polysaccharide (CPS) modulation (Wu et al., 2011).

The role of the capsule (CPS) in Klebsiella species

The initial downregulation of capsule gene expression is necessary for the bacteria to attach the host cells (Clements et al., 2008). The Klebsiella capsule is considered one of the major virulence effectors. The CPS cluster is composed of many genes involved in its synthesis (wzi, wza, wzb, wzc, gnd, wca, cpsB, cpsG, and galF) and these are collectively known as the K-antigens (Paczosa and Mecsas, 2016). From the 77 described capsular (K) types, those of K1, K2, K4, and K5 are the most important and are associated with virulent isolates (Brisse et al., 2009). Capsule production can also be enhanced by the presence of transcriptional regulators rmpA and rmpA2 found on MGEs, among the common regulators of the CPS cluster (rcsA and rcsB) (Paczosa and Mecsas, 2016). Two other important genes involved in regulation include rfaH that encodes a transcriptional elongation factor and which was found to be important in a murine lung infection model (Bachman et al., 2015), and the htrA gene that codes for a serine protease involved in the capsule formation (Cortés et al., 2002). The complete mechanism by which CPS is regulated remains to be further elucidated.

Infection of epithelial cells

To infect internal organs, Klebsiella must cross the epithelial barrier (Fig. 1B). There are two different routes used to enter epithelial cells: the intracellular route in which the pathogen invades nonphagocytic epithelial cells, altering the host cytoskeleton (Cossart and Sansonetti, 2004); and the paracellular pathway, which involves the disruption of the tight junctions, allowing the pathogen to pass between epithelial cells (Okuda et al., 2010; Backert et al., 2013). M cells are also considered to be an entry route to cross the epithelial barrier. These special cells are responsible for presenting foreign antigens and intact bacterial cells to the macrophages and dendritic cells (Peterson and Artis, 2014). When the glycoprotein GP2 present on the surface of the cells binds to antigens or FimH, as it happens with E. coli or Salmonella, it activates transport across the epithelial barrier for presentation to the phagocytic cells (Hase et al., 2009).

K. pneumoniae has been shown to invade and replicate inside a nonphagocytic epithelial cell in an in vivo model and to translocate in vitro across the epithelial monolayer without damaging the integrity or permeability of the barrier (Hsu et al., 2015). The mechanism by which Klebsiella initiates the invasion is still unresolved; however, data suggest that Klebsiella interacts with the glycans in the epithelial cell through the type II secretion system referred to as PulA (Tomás et al., 2015). Through this mechanism, Klebsiella is able to reduce the inflammatory response by avoiding the TLR2–TLR4–MyD88 pathway. Toll-like receptors (TLR) are the common mechanism for bacterial recognition in epithelial cells. Normally, when activated, this pathway will trigger the activation of the nuclear factor-κB (NF-κB), which induces the activation and release of cytokines (Moranta et al., 2010; Peterson and Artis, 2014). PulA interaction with the epithelial cell causes the activation of the Rho GTPase, Cdc42, and the EGF–PI3K–Akt pathway, inducing cellular cytoskeleton rearrangements, thereby allowing the bacterium to translocate (Frank et al., 2013; Hsu et al., 2015). This mechanism is also used by Salmonella species and Shigella species, wherein a type III secretion system deploys effector proteins into the host cell that will cause structural rearrangements, thereby facilitating the microorganism to be internalized (Cossart and Sansonetti, 2004). Klebsiella polysaccharides play an important role during the infection mechanism. Apart from antimicrobial defence, CPS also plays a part in interfering with the TLRs on the cell surface during infection (Moranta et al., 2010). It appears that CPS might have a dual role by activating the EGF–PI3K–Akt and inhibiting the Rac1 pathway (Regueiro et al., 2011; Frank et al., 2013). This inhibition causes an increased expression of NOD1, which in turn upregulates the expression of CYLD and MKP-1, allowing Klebsiella to highjack two host systems used by the epithelial cells to return to homeostasis, the NF-κB and the MAPKs pathways, respectively (Regueiro et al., 2011). The LPS O-polysaccharide and the first glucose of the LPS core also aid in dampening down the inflammatory response of the epithelial cell by disturbing TLR recognition of Klebsiella, attenuating the activation of NF-κB (Tomás et al., 2015). Klebsiella is able to alter its lipid A composition in the presence of different tissues in a PhoPQ-regulated, LpxO-dependent manner (Llobet et al., 2015) and by ramA activation (De Majumdar et al., 2015).

The iron acquisition system in Klebsiella species

When inside the epithelial cell and in plasma, Klebsiella encounters an extreme environment in terms of iron availability. Iron is an essential co-factor for bacterial growth and replication and it has been implicated in virulence (Bialek-Davenet et al., 2014). One of the host's innate mechanisms when infected is to sequester the readily available iron to restrict the growth of the pathogen. In plasma, transferrin is responsible for transporting iron, thereby limiting its availability. In the case of bacterial infection, this system can be improved by shifting the binding of iron to lactoferrin (Bullen et al., 1972). To cope with the low levels of iron, Klebsiella is able to synthesize and secrete small iron-scavenging molecules known as siderophores, which acquire and deliver host iron to the bacterium, resulting in bacterial growth. In addition, siderophores also induce cellular stress responses related to the stabilization of the master transcription factor hypoxia-inducible factor-1α, ultimately resulting in bacterial dissemination to the spleen (Holden et al., 2016). Together with the two ABC efflux systems, Kfu (Lee et al., 2016) and Sit (Sun et al., 2014), and the receptor tonB (Hsieh et al., 2008), these iron-scavenging mechanisms are required for full virulence (Russo et al., 2014; Holt et al., 2015).

Several siderophores are produced by Klebsiella species, including enterobactin, yersiniabactin, salmochelin, and aerobactin, and some of these are encoded on the chromosome, whereas others are commonly found in MGEs (Paczosa and Mecsas, 2016).

Enterobactin is the siderophore with highest affinity for free iron and is encoded on the chromosome by the genes entABCDEF (biosynthesis system) and fepABCDG (transport system) (Crichton, 2016). The action of this siderophore can be neutralized by lipocalin-2 (Bachman et al., 2012). To counteract the action of the host, Klebsiella and other Gram-negative bacteria incorporate other siderophores to scavenge iron from the host. Yersiniabactin was initially identified in Yersinia species as part of the high pathogenic island (Carniel, 1999). This siderophore is encoded by the irp cluster and is associated with invasive Klebsiella infections, being one of the most prevalent virulence loci identified in hvKP (Carniel, 2001; Bachman et al., 2011; Holt et al., 2015). Although yersiniabactin is not affected by the action of lipocalin-2, this siderophore is unable to acquire iron in the presence of transferrin (Bachman et al., 2011).

Like yersiniabactin, aerobactin and salmochelin are also examples of an alternative system that is nonresponsive to the action of lipocalin-2. Aerobactin is a citrate-hydroxamate siderophore encoded by the genes iucABCD along with the cognate transporter iutA (Bailey et al., 2016) and salmochelin, an O-glycosylated form of enterobactin, encoded by the genes iroBCDE and the transporter iroN (Müller et al., 2009). Both are associated with hypervirulent Klebsiella infections (Hsieh et al., 2008; Russo et al., 2014; Holt et al., 2015) due to the presence of these clusters on virulence plasmids, which are often co-located with resistance markers such as the rmpA and kfuABC or the two-component regulatory system kvgAS (Nassif and Sansonetti, 1986; Darfeuille-Michaud et al., 1992; Chen et al., 2004; Wu et al., 2009; Russo et al., 2015). The presence of such siderophores increases the infection potential of K. pneumoniae.

Resistance/evasion to host immune system

Human plasma carries many proteins capable of binding to pathogen-expressed antigens and that subsequently activate the Complement system. The Complement system is also part of the innate defence mechanism of the host. The LPS and outer membrane proteins (OMP) are the primary targets (Albertí et al., 1993, 1996). Klebsiella CPS once more plays an important role as a barrier preventing the binding of antibodies or the C1q by masking the epitopes present in the bacterial cell wall, thereby blocking its activation (de Astorza et al., 2004; Lin et al., 2012; Lee et al., 2014).

OMPs are the main target of the membrane attack complex (MAC). LPS plays an important role in stabilizing the OMPs by protecting them from the external environment (Needham and Trent, 2013). Modifications in the LPS structure have been shown to cause changes in serum resistance in Klebsiella (Williams et al., 1983; Ciurana and Tomás, 1987). The same susceptibility to serum was detected when the transcriptional elongation factor rfaH, responsible for LPS regulation, was deleted (Bachman et al., 2015). Klebsiella can undergo several modifications in the LPS to evade host immune detection (Llobet et al., 2015). The elongation of the O-LPS leads to the deposition of C3b further away from the cell wall, preventing the proper localization of the MAC on the bacterial surface, resulting in reduced pore formation (Merino et al., 1992).

Another strategy used by Klebsiella is the production and secretion of LPS-containing outer membrane vesicles into the environment to absorb proteins of the Complement system, preventing the subsequent deposition of these proteins onto the bacteria surface (Lee et al., 2012; Turner et al., 2016).

Although all these mechanisms help bacteria to avoid macrophage detection inevitably, macrophage engulfment will take place either by presentation of antigens by M cells or by direct capture in the plasma. Nonetheless, Klebsiella seems to be adapted to survive macrophage hydrolytic activity, a feature also found in other pathogens (McClure and Schiller, 1996; Ruckdeschel et al., 1997; Richardson, 2015; Srikumar et al., 2015).

It has been suggested that Klebsiella might trigger the lipid rafts to favor internalization by the macrophage (Huang et al. 2013). Klebsiella is able to survive inside the macrophage in the Klebsiella-containing vacuole (KCV) by deviating from the canonical endocytic pathway through manipulation of the PI3K–Akt–Rab14 (Fig. 1C). Interestingly, the KCV was able to acquire the markers of the endocytic pathway EEA1 and Lamp1. By manipulating this pathway, Klebsiella can control phagosome maturation by blocking the fusion of the KCV with the lysosome (Cano et al., 2015). The acidic pH of the KCV triggers the downregulation of the CPS, possibly due to the high metabolic burden to maintain such a system, proving that the presence of the capsule is not necessary for survival inside the macrophage (Cano et al., 2015). The VF MgtC used by Salmonella to inhibit the activity of the bacterial F₁F₀ ATP synthase, thereby disrupting proton translocation, allows the bacterium to adapt to the acidification of the vacuole and restores metabolic imbalances (Lee et al., 2013). The Klebsiella MgtC protein is very similar to a homolog in Salmonella; so it is speculated that this protein might have the same function (Belon and Blanc-Potard, 2016). Klebsiella is also able to induce programmed macrophage death. Nevertheless, the mechanism by which it happens is not fully elucidated. While some authors report features of cell death similar to apoptosis (Cano et al., 2015), others reported features of pyronecrosis by activation of NLRP3-dependent pathway (Willingham et al., 2009; Webster et al., 2010). One possible reason for such variability could be the different experimental conditions used. Further studies are required to clarify the cell death mechanism used by K. pneumoniae.

Conclusions

As a hitherto incompletely recognized zoonotic bacterium, K. pneumoniae is responsible for a wide range of human CA and HA infections, involving not only humans, animals, and environments but also the food chain. MDR and hypervirulent KpI (hvKP) strains have been increasingly evolving into distinct CGs, demonstrating a broad capacity to infect host cells, and rendering infection by these strains, to be very challenging to treat. MGEs carrying MDR or virulence determinants contribute to the potential for rapid dissemination of K. pneumoniae. Continued studies on K. pneumoniae deciphering its mechanism of resistance, pathogenicity, physiology, and interactions with host tissues should provide insights into useful strategies to combat this pathogen of importance to human health.

Footnotes

Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

References

Aires

CAM

, da Conceição-Neto

, Tavares e Oliveira

, Dias

, Montezzi

, Picão

, et al. Emergence of the plasmid-mediated mcr-1 gene in clinical kpc-2-producing Klebsiella pneumoniae sequence type 392 in Brazil. Antimicrob Agents Chemother, 2017; 61:pii:e00317-17.

Ali Hassan Ahmed Al-Shammary. Epidemiological pattern of extended spectrum beta lactamase resistant Klebsiella pneumoniae (ESBL) isolated from mastitic milk in Baghdad. Curr Res Microbiol Biotechnol, 2017; 5:1170–1177.

Albertí

, Alvarez

, Merino

, Casado

, Vivanco

, Tomás

, et al. Analysis of complement C3 deposition and degradation on Klebsiella pneumoniae . Infect Immun, 1996; 64:4726–4732.

Albertí

, Marqués

, Camprubí

, Merino

, Tomás

, Vivanco

, et al. C1q binding and activation of the complement classical pathway by Klebsiella pneumoniae outer membrane proteins. Infect Immun, 1993; 61:852–860.

Alcántar-Curiel

, Blackburn

, Saldaña

, Gayosso-Vázquez

, Iovine

, De la Cruz

, et al. Multi-functional analysis of Klebsiella pneumoniae fimbrial types in adherence and biofilm formation. Virulence, 2013; 4:129–138.

Al-Kharousi

, Guizani

, Al-Sadi

, Al-Bulushi

, Shaharoona

. Hiding in fresh fruits and vegetables: Opportunistic pathogens may cross geographical barriers. Int J Microbiol, 2016; 2016:4292417.

Amosun

, Olatoye

, Adetosoye

. Antimicrobial resistance in Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa isolated from milk of dairy cows in three Nigerian cities. Nig Vet J, 2012; 617–623.

Anes

, McCusker

MPMP

, Fanning

, Martins

. The ins and outs of RND efflux pumps in Escherichia coli . Front Microbiol, 2015; 6:587.

Argudín

, Deplano

, Meghraoui

, Dodémont

, Heinrichs

, Denis

, et al. Bacteria from animals as a pool of antimicrobial resistance genes. Antibiotics, 2017; 6:12.

10.

Babu

, Murali

, Batra

. Incidence of Klebsiella pneumoniae subsp. pneumoniae in food and environmental samples isolated from Mysore city, india and its antibiogram. Int J Pharma Biosci, 2015; 6:B635–B641.

11.

Bachman

, Breen

, Deornellas

, Mu

, Zhao

, Wu

, et al. Genome-wide identification of Klebsiella pneumoniae fitness genes during lung infection. mBio, 2015; 6:e00775-15.

12.

Bachman

, Lenio

, Schmidt

, Oyler

, Weiser

. Interaction of lipocalin 2, transferrin, and siderophores determines the replicative niche of Klebsiella pneumoniae during pneumonia. mBio, 2012; 3:e00224-11.

13.

Bachman

, Oyler

, Burns

, Caza

, Lepine

, Dozois

, et al. Klebsiella pneumoniae yersiniabactin promotes respiratory tract infection through evasion of lipocalin 2. Infect Immun, 2011; 79:3309–3316.

14.

Backert

, Boehm

, Wessler

, Tegtmeyer

, Friedman

, Neimann

, et al. Transmigration route of Campylobacter jejuni across polarized intestinal epithelial cells: transcellular or both?. Cell Commun Signal, 2013; 11:72.

15.

Badri

, Ibrahim

, Mohamed

, Garbi

, Kabbashi

, Arbab

. Prevalence of extended spectrum beta lactamase (ESBL) producing Escherichia coli and Klebsiella pneumoniae isolated from raw milk. Mol Biol Open Access, 2017; 7:1000201.

16.

Bagley

ST.

Habitat association of Klebsiella species. Infect Control, 1985; 6:52–58.

17.

Bailey

, Drake

, Grant

, Gulick

. Structural and functional characterization of aerobactin synthetase IucA from a hypervirulent pathotype of Klebsiella pneumoniae . Biochemistry, 2016; 55:3559–3570.

18.

Bales

, Renke

, May

, Shen

, Nelson

. Purification and characterization of biofilm-associated EPS exopolysaccharides from ESKAPE organisms and other pathogens. PLoS One, 2013; 8:e67950.

19.

Balestrino

, Ghigo

J-M

, Charbonnel

, Haagensen

JAJ

, Forestier

. The characterization of functions involved in the establishment and maturation of Klebsiella pneumoniae in vitro biofilm reveals dual roles for surface exopolysaccharides. Environ Microbiol, 2008; 10:685–701.

20.

Barati

, Ghaderpour

, Chew

, Bong

, Thong

, Chong

, et al. Isolation and characterization of aquatic-borne Klebsiella pneumoniae from tropical estuaries in Malaysia. Int J Environ Res Public Health, 2016; 13:426.

21.

Belon

, Blanc-Potard

A-B

. Intramacrophage survival for extracellular bacterial pathogens: MgtC as a key adaptive factor. Front Cell Infect Microbiol, 2016; 6:52.

22.

Barbier

, Rodrigues

, Depret

, Passet

, Gal

, Piveteau

, Brisse

. The ZKIR assay, a real-time PCR method for the detection of Klebsiella pneumoniae and closely related species in environmental samples. Appl Environ Microbiol, 2020; 86:e02711-19.

23.

Bassetti

, Righi

, Carnelutti

, Graziano

, Russo

. Multidrug-resistant Klebsiella pneumoniae: Challenges for treatment, prevention and infection control. Expert Rev Anti Infect Ther, 2018; 16:749–761.

24.

Berrazeg

, Diene

, Drissi

, Kempf

, Richet

, Landraud

, et al. Biotyping of multidrug-resistant Klebsiella pneumoniae clinical isolates from France and Algeria using MALDI-TOF MS. PLoS One, 2013; 8:e61428.

25.

Bhatt

, Ahir

, Koringa

, Jakhesara

, Rank

, Nauriyal

, et al. Milk microbiome signatures of subclinical mastitis-affected cattle analysed by shotgun sequencing. J Appl Microbiol, 2012; 112:639–650.

26.

Bialek-Davenet

, Criscuolo

, Ailloud

, Passet

, Jones

, Delannoy-Vieillard

, et al. Genomic definition of hypervirulent and multidrug-resistant Klebsiella pneumoniae clonal groups. Emerg Infect Dis, 2014; 20:1812–1820.

27.

Boddicker

, Anderson

, Jagnow

, Clegg

. Signature-tagged mutagenesis of Klebsiella pneumoniae to identify genes that influence biofilm formation on extracellular matrix material. Infect Immun, 2006; 74:4590–4597.

28.

Bowers

, Kitchel

, Driebe

, MacCannell

, Roe

, Lemmer

, de Man

, Rasheed

, Engelthaler

, Keim

, Limbago

. Genomic analysis of the emergence and rapid global dissemination of the clonal group 258 Klebsiella pneumoniae pandemic. PLoS One, 2015; 10:e0133727.

29.

Breurec

, Guessennd

, Timinouni

, Le

TAH

, Cao

, Ngandjio

, et al. Klebsiella pneumoniae resistant to third-generation cephalosporins in five African and two Vietnamese major towns: Multiclonal population structure with two major international clonal groups, CG15 and CG258. Clin Microbiol Infect, 2013; 19:349–355.

30.

Brisse

, Fevre

, Passet

, Issenhuth-Jeanjean

, Tournebize

, Diancourt

, et al. Virulent clones of Klebsiella pneumoniae: Identification and evolutionary scenario based on genomic and phenotypic characterization. PLoS One, 2009; 4:e4982.

31.

Brisse

, Grimont

PAD

. The genus Klebsiella taxonomic history and structure. Prokaryotes, 2006:159–196.

32.

Brisse

, Milatovic

, Fluit

, Verhoef

, Schmitz

. Epidemiology of quinolone resistance of Klebsiella pneumoniae and Klebsiella oxytoca in Europe. Eur J Clin Microbiol Infect Dis, 2000; 19:64–68.

33.

Brisse

, Passet

, Grimont

PAD

. Description of Klebsiella quasipneumoniae sp. nov., isolated from human infections, with two subspecies, Klebsiella quasipneumoniae subsp. quasipneumoniae subsp. nov. and Klebsiella quasipneumoniae subsp. similipneumoniae subsp. nov., and demonstration. Int J Syst Evol Microbiol, 2014; 64:3146–3152.

34.

Bródka

, Kozajda

, Buczyńska

, Szadkowska-Stańczyk

. The variability of bacterial aerosol in poultry houses depending on selected factors. Int J Occup Med Environ Health, 2012; 25:281–293.

35.

Buckner

MMC

, Saw

HTH

, Osagie

, McNally

, Ricci

, Wand

, et al. Clinically relevant plasmid-host interactions indicate that transcriptional and not genomic modifications ameliorate fitness costs of Klebsiella pneumoniae carbapenemase-carrying plasmids. mBio, 2018; 9:e02303–e02317.

36.

Bullen

, Rogers

, Griffiths

. Iron binding proteins and infection. Br J Haematol, 1972; 23:389–392.

37.

Calbo

, Freixas

, Xercavins

, Riera

, Nicolás

, Monistrol

, et al. Foodborne nosocomial outbreak of shv1 and ctx-m-15-producing Klebsiella pneumoniae: Epidemiology and control. Clin Infect Dis, 2011; 52:743.

38.

Campos

, Albiero

, Ecker

, Kuroda

, Meirelles

, Polato

, et al. Outbreak of Klebsiella pneumoniae carbapenemase-producing K. pneumoniae: A systematic review. Am J Infect Control, 2016; 44:1374–1380.

39.

Cano

, March

, Insua

, Aguiló

, Llobet

, Moranta

, et al. Klebsiella pneumoniae survives within macrophages by avoiding delivery to lysosomes. Cell Microbiol, 2015; 17:1537–1560.

40.

Carey

, Heidari-Torkabadi

. New techniques in antibiotic discovery and resistance: Raman spectroscopy. Ann N Y Acad Sci, 2015; 1354:67–81.

41.

Carniel

The Yersinia high-pathogenicity island. Int Microbiol, 1999; 2:161–167.

42.

Carniel

The Yersinia high-pathogenicity island: An iron-uptake island. Microbes Infect, 2001; 3:561–569.

43.

Casewell

, Phillips

. Food as a source of Klebsiella species for colonisation and infection of intensive care patients. J Clin Pathol, 1978; 31:845–849.

44.

Cerdeira

, Silva

, Fernandes

, et al. Draft genome sequence of a CTX-M-15-producing Klebsiella pneumoniae sequence type 340 (clonal complex 258) isolate from a food-producing animal. J Glob Antimicrob Resist, 2016; 7:67–68.

45.

Chen

K-M

, Chiang

M-K

, Wang

, Ho

H-C

, Lu

M-C

, Lai

Y-C

. The role of pgaC in Klebsiella pneumoniae virulence and biofilm formation. Microb Pathog, 2014a;77:89–99.

46.

Chen

, Mathema

, Chavda

, DeLeo

, Bonomo

, Kreiswirth

. Carbapenemase-producing Klebsiella pneumoniae: Molecular and genetic decoding. Trends Microbiol, 2014b;22:686–696.

47.

Chen

Y-T

, Chang

H-Y

, Lai

Y-C

, Pan

C-C

, Tsai

S-F

, Peng

H-L

. Sequencing and analysis of the large virulence plasmid pLVPK of Klebsiella pneumoniae CG43. Gene, 2004; 337:189–198.

48.

Cheong

, Kim

, Kang

, Choi

, Lee

. Rapid label-free identification of Klebsiella pneumoniae antibiotic resistant strains by the drop-coating deposition surface-enhanced Raman scattering method. Spectrochim Acta A Mol Biomol Spectrosc, 2017; 183:53–59.

49.

Choi

M-J

, Kim

, Ko

. Pathways regulating the pbgP operon and colistin resistance in Klebsiella pneumoniae strains. J Microbiol Biotechnol, 2016; 26:1620–1628.

50.

Chou

H-C

, Lee

C-Z

, Ma

L-C

, Fang

C-T

, Chang

S-C

, Wang

J-T

. Isolation of a chromosomal region of Klebsiella pneumoniae associated with allantoin metabolism and liver infection. Infect Immun, 2004; 72:3783–3792.

51.

Chung

, Lee

, Park

, et al. Fecal carriage of serotype K1 Klebsiella pneumoniae ST23 strains closely related to liver abscess isolates in Koreans living in Korea. Eur J Clin Microbiol Infect Dis, 2011; 31:481–486.

52.

Ciurana

, Tomás

. Role of lipopolysaccharide and complement in susceptibility of Klebsiella pneumoniae to nonimmune serum. Infect Immun, 1987; 55:2741–2746.

53.

Clements

, Gaboriaud

, Duval

JFL

, Farn

, Jenney

, Lithgow

, et al. The major surface-associated saccharides of Klebsiella pneumoniae contribute to host cell association. PLoS One, 2008; 3:e3817.

54.

Cortés

, de Astorza

, Benedí

, Albertí

. Role of the htrA gene in Klebsiella pneumoniae virulence. Infect Immun, 2002; 70:4772–4776.

55.

Cossart

, Sansonetti

. Bacterial invasion: The paradigms of enteroinvasive pathogens. Science, 2004; 304:242–248.

56.

Coudeyras

, Nakusi

, Charbonnel

, Forestier

. A tripartite efflux pump involved in gastrointestinal colonization by Klebsiella pneumoniae confers a tolerance response to inorganic acid. Infect Immun, 2008; 76:4633–4641.

57.

Crichton

Iron acquisition by pathogens. In: Iron Metabolism. Chichester, United Kingdom: John Wiley and Sons, Ltd., 2016, pp. 120–154.

58.

Curiao

, Morosini

, Ruiz-Garbajosa

, Robustillo

, Baquero

, Coque

, et al. Emergence of bla KPC-3-Tn4401a associated with a pKPN3/4-like plasmid within ST384 and ST388 Klebsiella pneumoniae clones in Spain. J Antimicrob Chemother, 2010; 65:1608–1614.

59.

Cuzon

, Naas

, Lesenne

, Benhamou

, Nordmann

, Queenan

, et al. Plasmid-mediated carbapenem-hydrolysing OXA-48 beta-lactamase in Klebsiella pneumoniae from Tunisia. Int J Antimicrob Agents, 2010; 36:91–93.

60.

Darfeuille-Michaud

, Jallat

, Aubel

, Sirot

, Rich

, Sirot

, et al. R-plasmid-encoded adhesive factor in Klebsiella pneumoniae strains responsible for human nosocomial infections. Infect Immun, 1992; 60:44–55.

61.

Davis

, Price

. Recent research examining links among Klebsiella pneumoniae from food, food animals, and human extraintestinal infections. Curr Environ Health Rep 2016:1–8.

62.

Davis

, Waits

, Nordstrom

, Weaver

, Aziz

, Gauld

, et al. Intermingled Klebsiella pneumoniae populations between retail meats and human urinary tract infections. Clin Infect Dis, 2015; 61:892–899.

63.

de Astorza

, Cortés

, Crespí

, Saus

, Rojo

, Albertí

. C3 promotes clearance of Klebsiella pneumoniae by A549 epithelial cells. Infect Immun, 2004; 72:1767–1774.

64.

Decré

, Verdet

, Emirian

, Le Gourrierec

, Petit

J-C

, Offenstadt

, et al. Emerging severe and fatal infections due to Klebsiella pneumoniae in two university hospitals in France. J Clin Microbiol, 2011; 49:3012–3014.

65.

Dedeić-Ljubović

, Hukić

. Catheter-related urinary tract infection in patients suffering from spinal cord injuries. Bosn J Basic Med Sci, 2009; 9:2–9.

66.

De Majumdar

, Yu

, Fookes

, McAteer

, Llobet

, Finn

, et al. Elucidation of the RamA regulon in Klebsiella pneumoniae reveals a role in LPS regulation. PLoS Pathog, 2015; 11:e1004627.

67.

Diab

, Hamze

, Bonnet

, Saras

, Madec

, Haenni

. OXA-48 and CTX-M-15 extended-spectrum beta-lactamases in raw milk in Lebanon: Epidemic spread of dominant Klebsiella pneumoniae clones. J Med Microbiol, 2017; 66:1688–1691.

68.

Diancourt

, Passet

, Verhoef

, Grimont

PAD

, Brisse

. Multilocus sequence typing of Klebsiella pneumoniae nosocomial isolates. J Clin Microbiol, 2005; 43:4178–4182.

69.

Di Martino

, Bertin

, Girardeau

, Livrelli

, Joly

, Darfeuille-Michaud

. Molecular characterization and adhesive properties of CF29K, an adhesin of Klebsiella pneumoniae strains involved in nosocomial infections. Infect Immun, 1995; 63:4336–4344.

70.

Di Martino

, Livrelli

, Sirot

, Joly

, Darfeuille-Michaud

, Martino

PDI

, et al. A new fimbrial antigen harbored by CAZ-5/SHV-4-producing Klebsiella pneumoniae strains involved in nosocomial infections. Infect Immun, 1996; 64:2266–2273.

71.

El-Mahallawy

, Hassan

, El-Wakil

, Moneer

. Bacteremia due to ESKAPE pathogens: An emerging problem in cancer patients. J Egypt Natl Canc Inst, 2016; 28:157–162.

72.

Evans

, Xie

. 2008. Coherent anti-stokes Raman scattering microscopy: Chemical imaging for biology and medicine. Annu Rev Anal Chem, 2008; 1:883–909.

73.

Fang

, Lai

, Yi

, Hsueh

, Liu

, Chang

. Klebsiella pneumoniae genotype K1: An emerging pathogen that causes septic ocular or central nervous system complications from pyogenic liver abscess. Clin Infect Dis, 2007; 45:284–293.

74.

Fevre

, Passet

, Weill

F-X

, Grimont

PAD

, Brisse

. Variants of the Klebsiella pneumoniae OKP chromosomal beta-lactamase are divided into two main groups, OKP-A and OKP-B. Antimicrob Agents Chemother, 2005; 49:5149–5152.

75.

Fierer

, Walls

, Chu

. Recurring Klebsiella pneumoniae pyogenic liver abscesses in a resident of San Diego, California, due to a K1 strain carrying the virulence plasmid. J Clin Microbiol, 2011; 49:4371–4373.

76.

Frank

, Reguerio

, Rother

, Moranta

, Maeurer

, Garmendia

, et al. Klebsiella pneumoniae targets an EGF receptor-dependent pathway to subvert inflammation. Cell Microbiol, 2013; 15:1212–1233.

77.

Fung

C-P

, Lin

Y-T

, Lin

J-C

, Chen

T-L

, Yeh

K-M

, Chang

F-Y

, et al. Klebsiella pneumoniae in gastrointestinal tract and pyogenic liver abscess. Emerg Infect Dis, 2012; 18:1322–1325.

78.

García-Cobos

, Köck

, Mellmann

, Frenzel

, Friedrich

, Rossen

JWA

. Molecular typing of Enterobacteriaceae from pig holdings in North-Western Germany reveals extended-spectrum and AmpC β-lactamases producing but no carbapenem resistant ones. PLoS One, 2015; 10:e0134533.

79.

Gessain

, Tsai

Y-H

, Travier

, Bonazzi

, Grayo

, Cossart

, et al. PI3-kinase activation is critical for host barrier permissiveness to Listeria monocytogenes . J Exp Med, 2015; 212:165–183.

80.

Ghafur

, Shankar

, GnanaSoundari

, Venkatesan

, Mani

, Thirunarayanan

, Veeraraghavan

. Detection of chromosomal and plasmid-mediated mechanisms of colistin resistance in Escherichia coli and Klebsiella pneumoniae from Indian food samples. J Glob Antimicrob Resist, 2019; 16:48–52.

81.

Guyot

, Biran

, Doit

, Moissenet

, Guillard

, Brasme

, Courroux

, Maquelin

, Leeuwen

, VuThien

, Aujard

, Champs CD & Bingen

. Raman spectroscopic analysis of the clonal and horizontal spread of CTX-M-15-producing Klebsiella pneumoniae in a neonatal intensive care unit. Eur J Clin Microbiol Infect Dis. 2012; 31:2827–2834.

82.

Gundogan

, Citak

, Yalcin

. Virulence properties of extended spectrum β-lactamase-producing Klebsiella species in meat samples. J Food Prot, 2011; 74:559–564.

83.

, Dong

, Zheng

, Lin

, Huang

, Wang

, Chan

, Shu

, Yu

, Zhang

, Chen

. A fatal outbreak of ST11 carbapenem-resistant hypervirulent Klebsiella pneumoniae in a Chinese hospital: a molecular epidemiological study. Lancet Infect Dis, 2018; 18:37–46.

84.

, Huang

, Ma

, Zhou

, Fang

, Cai

, et al. Detection of colistin resistance gene mcr-1 in hypervirulent Klebsiella pneumoniae and Escherichia coli isolates from an infant with diarrhea in China. Antimicrob Agents Chemother, 2016; 60:5099–5100.

85.

Hamasha

, Mohaidat

, Putnam

, Woodman

, Palchaudhuri

, Rehse

. Sensitive and specific discrimination of pathogenic and nonpathogenic Escherichia coli using Raman spectroscopy-a comparison of two multivariate analysis techniques. Biomed Opt Express, 2013; 4:481–489.

86.

Hammad

, Shimamoto

. Emergence of a novel β-lactamase in food of animal origin: characterization of SHV-111, from a cheese isolate of Klebsiella pneumoniae, with an amino acid substitution in a strictly conserved position (P174S) in the Ω-loop. J Med Microbiol, 2017; 66:96–97.

87.

Hartantyo

SHP

, Chau

, Koh

, Yap

, Yi

, Cao

DYH

, GutiÉrrez

, Ng

. Foodborne Klebsiella pneumoniae: Virulence Potential, Antibiotic Resistance, and Risks to Food Safety. J Food Prot, 2020; 83:1096–1103.

88.

Haryani

, Noorzaleha

, Fatimah

, Noorjahan

, Patrick

, Shamsinar

, Laila

RAS

, & Son

. Incidence of Klebsiella pneumonia in street foods sold in Malaysia and their characterization by antibiotic resistance, plasmid profiling, and RAPD-PCR analysis. Food Control, 2007; 18:847–853.

89.

Hase

, Kawano

, Nochi

, Pontes

, Fukuda

, Ebisawa

, et al. Uptake through glycoprotein 2 of FimH(+) bacteria by M cells initiates mucosal immune response. Nature, 2009; 462:226–230.

90.

Hassan

, Farooqui

, Khan

, Kazmi

. Microbial contamination of raw meat and its environment in retail shops in Karachi, Pakistan. J Infect Dev Ctries, 2010; 4:382–388.

91.

Hatheway

, Farmer

, 3rd. Clostridium perfringens or Klebsiella pneumoniae as the cause of a food-borne outbreak. J Clin Microbiol, 1991; 29:415–416.

92.

Hennequin

, Robin

. Correlation between antimicrobial resistance and virulence in Klebsiella pneumoniae . Eur J Clin Microbiol Infect Dis, 2016; 35:1–9.

93.

Hiroi

, Yamazaki

, Harada

, Takahashi

, Iida

, Noda

, et al. Prevalence of extended-spectrum β-lactamase-producing Escherichia coli and Klebsiella pneumoniae in food-producing animals. J Vet Med Sci, 2012; 74:189–195.

94.

Hlaing

, Wood

, McNaughton

, Rood

, Fox

, Augustin

. Vibrational spectroscopy combined with transcriptomic analysis for investigation of bacterial responses towards acid stress. Appl Microbiol Biotechnol, 2018; 102:333–343.

95.

Holden

, Breen

, Houle

, Dozois

, Bachman

. Klebsiella pneumoniae siderophores induce inflammation, bacterial dissemination, and HIF-1α stabilization during pneumonia. mBio, 2016; 7:e01397-16.

96.

Holt

, Wertheim

, Zadoks

, Baker

, Whitehouse

, Dance

, et al. Genomic analysis of diversity, population structure, virulence, and antimicrobial resistance in Klebsiella pneumoniae, an urgent threat to public health. Proc Natl Acad Sci U S A, 2015; 112:E3574–E3581.

97.

Hong

, Karanja

, Abutaleb

, Younis

, Zhang

, Seleem

, Cheng

. Antibiotic susceptibility determination within one cell cycle at single-bacterium level by stimulated Raman metabolic imaging. Anal Chem, 2018; 90:3737–3743.

98.

Horvath

, Hanny

. [Food poisoning caused by Klebsiella]. Orv Hetil, 1965; 106:1417–1418 (Article in German).

99.

Horváth

, Hanny

, Pethes

. [Mass food poisoning with bacteria of the Klebsiella group]. Zbl Bakt I Abit Orig, 1964; 193:191–195 (Article in German).

100.

Horváth

, Lantos

, Fekete

, Marjai

. Phage types, antibiograms and R-plasmids of Klebsiella and Enterobacter isolated from hospital environment and food. Acta Microbiol Hung, 1986; 33:285–294.

101.

Hsieh

P-F

, Hsu

C-R

, Chen

C-T

, Lin

T-L

, Wang

J-T

. The Klebsiella pneumoniae YfgL (BamB) lipoprotein contributes to outer membrane protein biogenesis, type-1 fimbriae expression, anti-phagocytosis, and in vivo virulence. Virulence, 2016; 7:587–601.

102.

Hsieh

P-F

, Lin

T-L

, Lee

C-Z

, Tsai

S-F

, Wang

J-T

. Serum-induced iron-acquisition systems and TonB contribute to virulence in Klebsiella pneumoniae causing primary pyogenic liver abscess. J Infect Dis, 2008; 197:1717–1727.

103.

Hsieh

P-F

, Liu

J-Y

, Pan

Y-J

, Wu

M-C

, Lin

T-L

, Huang

Y-T

, et al. Klebsiella pneumoniae peptidoglycan-associated lipoprotein and murein lipoprotein contribute to serum resistance, antiphagocytosis, and proinflammatory cytokine stimulation. J Infect Dis, 2013; 208:1580–1589.

104.

Hsu

, Lin

, Chen

, Chou

, Wang

. The role of Klebsiella pneumoniae rmpA in capsular polysaccharide synthesis and virulence revisited. Microbiology, 2011; 157(Pt 12):3446–3457.

105.

Hsu

C-RR

, Pan

Y-JJ

, Liu

J-YY

, Chen

C-TT

, Lin

T-LL

, Wang

J-T

, et al. Klebsiella pneumoniae translocates across the intestinal epithelium via rho GTPase-and phosphatidylinositol 3-kinase/Akt-dependent cell invasion. Infect Immun, 2015; 83:769–779.

106.

Huang

, Weaver

, Wu

, Li

, Gao

, Fan

, et al. Lipid-based signaling modulates DNA repair response and survival against Klebsiella pneumoniae infection in host cells and in mice. Am J Respir Cell Mol Biol, 2013; 49:798–807.

107.

Huang

T-W

, Wang

J-T

, Lauderdale

T-L

, Liao

T-L

, Lai

J-F

, Tan

M-C

, et al. Complete sequences of two plasmids in a blaNDM-1-positive Klebsiella oxytoca isolate from Taiwan. Antimicrob Agents Chemother, 2013; 57:4072–4076.

108.

Hudson

, Bent

, Meagher

, Williams

. Resistance determinants and mobile genetic elements of an NDM-1-encoding Klebsiella pneumoniae strain. PLoS One, 2014; 9:e99209.

109.

Ibenyassine

, Mhand

, Karamoko

, Anajjar

, Chouibani

, Ennaji

. Bacterial pathogens recovered from vegetables irrigated by wastewater in Morocco. J Environ Health, 2007; 69:47–51.

110.

Imhoff

JF.

Enterobacteriales. In: Brenner

, Krieg

, Staley

, Garrity

, Boone

, De Vos

, et al., eds. Bergey's Manual^® of Systematic Bacteriology. Boston, MA: Springer US, 2005, pp. 587–850.

111.

Jagnow

, Clegg

. Klebsiella pneumoniae MrkD-mediated biofilm formation on extracellular matrix- and collagen-coated surfaces. Microbiology, 2003; 149:2397–2405.

112.

Janda

JM.

The genus Klebsiella: An ever-expanding panorama of infections, disease-associated syndromes, and problems for clinical microbiologists. Clin Microbiol Case Rep, 2015; 1:1–7.

113.

Jin

, Shao

, Li

, Fan

, Bai

, Wang

. Outbreak of multidrug resistant ndm-1-producing Klebsiella pneumoniae from a neonatal unit in shandong province, china. PLoS One, 2015; 10:e0119571.

114.

Johnson

, Murphy

, Sippy

, Johnson

, Clegg

. Type 3 fimbriae and biofilm formation are regulated by the transcriptional regulators MrkHI in Klebsiella pneumoniae . J Bacteriol, 2011; 193:3453–3460.

115.

Kabha

, Nissimov

, Athamna

, Keisari

, Parolis

, et al. Relationships among capsular structure, phagocytosis, and mouse virulence in Klebsiella pneumoniae . Infect Immun, 1995; 63:847–852.

116.

Kanevsky-Mullarky

, Nedrow

, Garst

, Wark

, Dickenson

, Petersson-Wolfe

, et al. Short communication: Comparison of virulence factors in Klebsiella pneumoniae strains associated with multiple or single cases of mastitis. J Dairy Sci, 2014; 97:2213–2218.

117.

Karaiskos

, Giamarellou

. Multidrug-resistant and extensively drug-resistant Gram-negative pathogens: Current and emerging therapeutic approaches. Expert Opin Pharmacother, 2014; 15:1351–1370.

118.

Kaur

, Kaul

, Chhibber

. Enterotoxigenicity, klebocinogeny and antibiotic resistance pattern of food isolates of Klebsiella pneumoniae . Folia Microbiol (Praha), 1988; 33:500–506.

119.

Kazanji

, Klein

, Lohani

, Mertens

, Le

. A case of hypermucoviscous Klebsiella pneumoniae liver abscess syndrome in an Iraqi male. QJM, 2016; 109:493–494.

120.

Keynan

, Rubinstein

. The changing face of Klebsiella pneumoniae infections in the community. Int J Antimicrob Agents, 2007; 30:385–389.

121.

Kiddy

, Josse

, Griffin

. An outbreak of serious Klebsiella infections related to food blenders. J Hosp Infect, 1987; 9:191–193.

122.

Kieffer

, AiresdeSousa

, Nordmann

, Poirel

. High rate of mcr-1-producing Escherichia coli and Klebsiella pneumoniae among pigs, Portugal. Emerg Infect Dis, 2017; 23:2023–2029.

123.

Kim

, Chon

, Kim

, Seo

. Prevalence and characterization of extended-spectrum-β-lactamase-producing Escherichia coli and Klebsiella pneumoniae in ready-to-eat vegetables. Int J Food Microbiol, 2015; 207:83–86.

124.

Kim

S-H

, Wei

C-I

, Tzou

Y-M

, An

. Multidrug-resistant Klebsiella pneumoniae isolated from farm environments and retail products in Oklahoma. J Food Prot, 2005; 68:2022–2029.

125.

Kirk

, Nielsen

, Thorlacius-Ussing

, Nielsen

. Optimized cultivation of Campylobacter concisus from gut mucosal biopsies in inflammatory bowel disease. Gut Pathog, 2016; 8:27.

126.

Koovapra

, Bandyopadhyay

, Das

, Bhattacharyya

, Banerjee

, Mahanti

, et al. Molecular signature of extended spectrum β-lactamase producing Klebsiella pneumoniae, isolated from bovine milk in Eastern and North-Eastern India. Infect Genet Evol, 2016; 44:395–402.

127.

Lai

Y-C

, Lu

M-C

, Hsueh

P-R

. Hypervirulence and carbapenem resistance: Two distinct evolutionary directions that led high-risk Klebsiella pneumoniae clones to epidemic success, Expert Rev Mol Diagn, 2019; 19:825–837.

128.

Lam

, Wyres

, Duchêne

, Wick

, Judd

, Gan

, Hoh

, Archuleta

, Molton

, Kalimuddin

, Koh

, Passet

, Brisse

, Holt

. Population genomics of hypervirulent Klebsiella pneumoniae clonal-group 23 reveals early emergence and rapid global dissemination. Nat Commun, 2018; 9:2703.

129.

Langstraat

, Bohse

, Clegg

. Type 3 fimbrial shaft (MrkA) of Klebsiella pneumoniae, but not the fimbrial adhesin (MrkD), facilitates biofilm formation. Infect Immun, 2001; 69:5805–5812.

130.

Leangapichart

, Gautret

, Brouqui

, Mimish

, Raoult

, Rolain

J-M

. Acquisition of mcr-1 plasmid-mediated colistin resistance in Escherichia coli and Klebsiella pneumoniae during Hajj 2013 and 2014. Antimicrob Agents Chemother, 2016; 60:6998–6999.

131.

Lee

C-H

, Chang

C-C

, Liu

J-W

, Chen

R-F

, Yang

. Sialic acid involved in hypermucoviscosity phenotype of Klebsiella pneumoniae and associated with resistance to neutrophil phagocytosis. Virulence, 2014; 5:673–679.

132.

Lee

E-J

, Pontes

, Groisman

, Alix

, Blanc-Potard

, Allison

, et al. A bacterial virulence protein promotes pathogenicity by inhibiting the bacterium's own F1Fo ATP synthase. Cell, 2013; 154:146–156.

133.

Lee

, Burgess

. Treatment of Klebsiella pneumoniae carbapenemase (KPC) infections: A review of published case series and case reports. Ann Clin Microbiol Antimicrob, 2012; 11:32.

134.

Lee

, Lee

, Jun

, Choi

, Kim

, et al. Klebsiella pneumoniae secretes outer membrane vesicles that induce the innate immune response. FEMS Microbiol Lett, 2012; 331:17–24.

135.

Lee

, Molton

, Wyres

, Gorrie

, Wong

, Hoh

, et al. Differential host susceptibility and bacterial virulence factors driving Klebsiella liver abscess in an ethnically diverse population. Sci Rep, 2016; 6:29316.

136.

, Yang

, Miao

, Chavda

, Mediavilla

, Xie

. Complete sequences of mcr-1-harboring plasmids from extended-spectrum-β-lactamase- and carbapenemase-producing Enterobacteriaceae. Antimicrob Agents Chemother, 2016; 60:4351–4354.

137.

, Lim

. A novel large plasmid carrying multiple β-lactam resistance genes isolated from a Klebsiella pneumoniae strain. J Appl Microbiol, 2000; 88:1038–1048.

138.

, Plésiat

, Nikaido

. The challenge of efflux-mediated antibiotic resistance in Gram-negative bacteria. Clin Microbiol Revi, 2015; 28:337–418.

139.

Lin

, Yang

, Peng

, Li

, Tsai

, et al. Amino acid substitutions of MagA in Klebsiella pneumoniae affect the biosynthesis of the capsular polysaccharide. PLoS One, 2012; 7:e46783.

140.

Liu

, Zheng

, Jin

, Chen

, Yang

. VFDB 2019: A comparative pathogenomic platform with an interactive web interface. Nucleic Acids Res, 2019; 47:D687–D692.

141.

Liu

, Ye

, Li

, et al. Identification and characterization of an antibacterial type VI secretion system in the carbapenem-resistant strain Klebsiella pneumoniae HS11286. Front Cell Infect Microbiol, 2017; 7:442.

142.

Liu

, Lin

, Hung

, Liu

, Chen

, Huang

, Tsai

, Wang

, et al. A high speed detection platform based on surface-enhanced Raman scattering for monitoring antibiotic-induced chemical changes in bacteria cell wall. PLoS One, 2009; 4:e5470.

143.

Liu

, Wang

, Jiang

. An increasing prominent disease of Klebsiella pneumoniae liver abscess: Etiology, diagnosis, and treatment. Gastroenterol Res Pract, 2013; 2013:258514.

144.

Liu

, Zhang

, Cao

, Huang

, Shen

, Qin

. Two clinical strains of Klebsiella pneumoniae carrying plasmid-borne bla _IMP-4, bla _SHV-12, and armA isolated at a pediatric Center in Shanghai, China. Antimicrob Agents Chemother, 2009; 53:1642–1644.

145.

Liu

, Wang

, Walsh

, Yi

, Zhang

, Spencer

, et al. Emergence of plasmid-mediated colistin resistance mechanism MCR-1 in animals and human beings in China: A microbiological and molecular biological study. Lancet Infect Dis, 2016; 16:161–168.

146.

Llobet

, Martínez-Moliner

, Moranta

, Dahlström

, Regueiro

, Tomás

, et al. Deciphering tissue-induced Klebsiella pneumoniae lipid A structure. Proc Natl Acad Sci U S A, 2015; 112:E6369–E6378.

147.

Locatelli

, Pisoni

, Moroni

. Ctx-m1 ESBL-producing Klebsiella pneumoniae subsp. pneumoniae isolated from cases of bovine mastitis. J Clin Microbiol, 2010; 48:3822–3823.

148.

Long

, Linson

, Ojeda Saavedra

, Cantu

, Davis

, Brettin

, Olsen

. Whole-genome sequencing of a human clinical isolate of the novel species Klebsiella quasivariicola sp. nov. Genome Announc, 2017; 5:pii:e01057-17.

149.

Lorenz

, Wichmann

, Stöckel

, Rösch

, Popp

. Cultivation-free Raman spectroscopic investigations of bacteria. Trends Microbiol, 2017; 25:413–424.

150.

, Chen

, Chiang

, et al. Colibactin contributes to the hypervirulence of pks+ K1 CC23 Klebsiella pneumoniae in mouse meningitis infections. Front Cell Infect Microbiol, 2017; 7:103.

151.

, Wan

, Wang

, Gao

, Yang

, Partridge

, Wang

, Zong

, Doi

, Shen

, Jia

, Song

, Zhang

, Yang

, Huang

, Wang

, Liu

. Emergence of a plasmid-encoded resistance-nodulation-division efflux pump conferring resistance to multiple drugs, including tigecycline, in Klebsiella pneumoniae . mBio, 2020; 11:e02930-19.

152.

Maatallah

, Vading

, Kabir

, Bakhrouf

, Kalin

, Nauclér

, et al. Klebsiella variicola is a frequent cause of bloodstream infection in the Stockholm area, and associated with higher mortality compared to K. pneumoniae . PLoS One, 2014; 9:e113539.

153.

Markart

, Korfhagen

, Weaver

, Akinbi

. Mouse lysozyme M is important in pulmonary host defense against Klebsiella pneumoniae infection. Am J Respir Crit Care Med, 2004; 169:454–458.

154.

Maroncle

, Rich

, Forestier

. The role of Klebsiella pneumoniae urease in intestinal colonization and resistance to gastrointestinal stress. Res Microbiol, 2006; 157:184–193.

155.

Martínez-Romero

, Silva-Sanchez

, Barrios

, Rodríguez-Medina

, Martínez-Barnetche

, Téllez-Sosa

, et al. Draft genome sequences of Klebsiella variicola plant isolates. Genome Announc, 2015; 3:3–4.

156.

Mary

, Tahere

, Edmondson

, Brayson

, Diana

. Klebsiella species in hospital food and kitchens: A source of organisms in the bowel of patients. Epidemiol Infect, 1980; 84:97–101.

157.

Mathers

, Peirano

, Pitout

JDD

. The role of epidemic resistance plasmids and international high-risk clones in the spread of multidrug-resistant Enterobacteriaceae. Clin Microbiol Rev, 2015; 28:565–591.

158.

McClure

, Schiller

. Inhibition of macrophage phagocytosis by Pseudomonas aeruginosa rhamnolipids in vitro and in vivo. Curr Microbiol, 1996; 33:109–117.

159.

Mendes

, Novais Â, Campos

, Rodrigues

, Santos

, Antunes

. Mcr-1in carbapenemase-producing Klebsiella pneumoniae with hospitalized patients, Portugal, 2016–2017. Emerg Infect Dis, 2018; 4:762–766.

160.

Merino

, Camprubi

, Alberti

, Benedi

, Tomas

. Mechanisms of Klebsiella pneumoniae resistance to complement-mediated killing. Infect Immun, 1992; 60:2529–2535.

161.

Mohammed

, Abdel-Latef

, Abdel-Azeem

, El-Dakhly

. Ecological study on antimicrobial-resistant zoonotic bacteria transmitted by flies in cattle farms. Parasitol Res, 2016; 115:3889–3896.

162.

Montso

, Dlamini

, Kumar

, Ateba

. Antimicrobial resistance factors of extended-spectrum beta-lactamases producing Escherichia coli and Klebsiella pneumoniae isolated from cattle farms and raw beef in North-West province, South Africa. BioMed Res Int, 2019; 2019:4318306.

163.

Moranta

, Regueiro

, March

, Llobet

, Margareto

, Larrate

, et al. Klebsiella pneumoniae capsule polysaccharide impedes the expression of β-defensins by airway epithelial cells. Infect Immun, 2010; 78:1135–1146.

164.

Moussé

, Adjanohoun

, Sina

, Noumavo

, Ahissin

, Baba-Moussa

. Klebsiella pneumoniae isolated from street foods: Characterization for extended spectrum β-lactamases production and antibiotics resistance profile. Biochem Biotechnol Res, 2016; 4:27–37.

165.

Müller

, Valdebenito

, Hantke

. Salmochelin, the long-overlooked catecholate siderophore of Salmonella . Biometals, 2009; 22:691–695.

166.

Murphy

, Mortensen

, Krogfelt

, Clegg

. Role of Klebsiella pneumoniae type 1 and type 3 fimbriae in colonizing silicone tubes implanted into the bladders of mice as a model of catheter-associated urinary tract infections. Infect Immun, 2013; 81:3009–3017.

167.

Nadjar

Outbreak of Klebsiella pneumoniae producing transferable AmpC-type β-lactamase (ACC-1) originating from Hafnia alvei . FEMS Microbiol Lett, 2000; 187:35–40.

168.

Nang

, Morris

, Mcdonald

, Han

, Wang

, Strugnell

, et al. Fitness cost of mcr-1-mediated polymyxin resistance in Klebsiella pneumoniae . J Antimicrob Chemother, 2018; 73:1604–1610.

169.

Nassif

, Sansonetti

. Correlation of the virulence of Klebsiella pneumoniae K1 and K2 with the presence of a plasmid encoding aerobactin. Infect Immun, 1986; 54:603–608.

170.

Navon-Venezia

, Kondratyeva

, Carattoli

. Klebsiella pneumoniae: A major worldwide source and shuttle for antibiotic resistance. FEMS Microbiol Rev, 2017; 41:252–275.

171.

Nawaz

, Khan

, Tran

, Sung

, Khan

, Adamu

, et al. Isolation and characterization of multidrug-resistant Klebsiella spp. isolated from shrimp imported from Thailand. Int J Food Microbiol, 2012; 155:179–184.

172.

Needham

, Trent

. Fortifying the barrier: The impact of lipid A remodelling on bacterial pathogenesis. Nat Rev Microbiol, 2013; 11:467–481.

173.

Newtonfoot

, Snyman

, Maloba

MRB

, Whitelaw

. Plasmid-mediated mcr-1 colistin resistance in Escherichia coli and Klebsiella spp. clinical isolates from the Western Cape region of South Africa. Antimicrob Resist Infect Control, 2017; 6:78.

174.

Novović

, Trudić

, Brkić

, Vasiljević

, Kojić

, Medić

, et al. Molecular epidemiology of colistin-resistant, carbapenemase-producing Klebsiella pneumoniae in Serbia from 2013 to 2016. Antimicrob Agents Chemother, 2017; 61:e02550-16.

175.

Okuda

, Hayashi

, Okamoto

, Sawada

, Minagawa

, Yano

, et al. Translocation of Pseudomonas aeruginosa from the intestinal tract is mediated by the binding of ExoS to an Na,K-ATPase regulator, FXYD3. Infect Immun, 2010; 78:4511–4522.

176.

Osman

, Hassan

, Orabi

, Abdelhafez

AST

. Phenotypic, antimicrobial susceptibility profile and virulence factors of Klebsiella pneumoniae isolated from buffalo and cow mastitic milk. Pathog Glob Health, 2014; 108:191–199.

177.

Overdevest

ITMA

, Heck

, van der Zwaluw

, Huijsdens

, van Santen

, Rijnsburger

, et al. Extended-spectrum β-lactamase producing Klebsiella spp. in chicken meat and humans: A comparison of typing methods. Clin Microbiol Infect, 2014; 20:251–255.

178.

Paczosa

, Mecsas

. Klebsiella pneumoniae: Going on the offense with a strong defense. Microbiol Mol Biol Rev, 2016; 80:629–661.

179.

Padilla

, Llobet

, Doménech-Sánchez

, Martínez-Martínez

, Bengoechea

, Albertí

. Klebsiella pneumoniae AcrAB efflux pump contributes to antimicrobial resistance and virulence. Antimicrob Agents Chemother, 2010; 54:177–183.

180.

Pagani

, Perilli

, Migliavacca

, Luzzaro

, Amicosante

. Extended-spectrum TEM- and SHV-type beta-lactamase-producing Klebsiella pneumoniae strains causing outbreaks in intensive care units in Italy. Eur J Clin Microbiol Infect Dis, 2000; 19:765–772.

181.

Pahlow

, Meisel

, Cialla-May

, Weber

, Rösch

, Popp

. Isolation and identification of bacteria by means of Raman spectroscopy. Adv Drug Deliv Rev, 2015; 89:105–120.

182.

Palmeiro

, de Souza

, Schörner

, Passarelli-Araujo

, Grazziotin

, Vidal

, Venancio

, Dalla-Costa

. Molecular epidemiology of multidrug-resistant Klebsiella pneumoniae isolates in a Brazilian Tertiary Hospital. Front Microbiol, 2019; 10:1669.

183.

Passet

, Brisse

. Association of tellurite resistance with hypervirulent clonal groups of Klebsiella pneumoniae . J Clin Microbiol, 2015; 53:1380–1382.

184.

Pastagia

, Arumugam

, Alvarez

, Gonzalez

, Baldonedo

, Al. E, et al. Klebsiella pneumoniae liver abscesses in a public hospital in Queens, New York. Travel Med Infect Dis, 2008; 6:228–233.

185.

Pesesky

, Hussain

, Wallace

, Wang

, Andleeb

, Burnham

CAD

, et al. KPC and NDM-1 genes in related Enterobacteriaceae strains and plasmids from Pakistan and the United States. Emerg Infect Dis, 2015; 21:1034–1037.

186.

Peterson

, Artis

. Intestinal epithelial cells: Regulators of barrier function and immune homeostasis. Nat Rev Immunol, 2014; 14:141–153.

187.

Petrosillo

, Giannella

, Lewis

, Viale

. Treatment of carbapenem-resistant Klebsiella pneumoniae: The state of the art. Expert Rev Anti Infect Ther, 2013; 11:159–177.

188.

Podder

, Rogers

, Daley

, Keefe

, Whitney

, Tahlan

. Klebsiella species associated with bovine mastitis in Newfoundland. PLoS One, 2014; 9:1–5.

189.

Podschun

, Pietsch

, Höller

, Ullmann

. Incidence of Klebsiella species in surface waters and their expression of virulence factors. Appl Environ Microbiol, 2001; 67:3325–3327.

190.

Podschun

, Ullmann

. Klebsiella spp. as nosocomial pathogens: Epidemiology, taxonomy, typing methods, and pathogenicity factors. Clin Microbiol Rev, 1998; 11:589–603.

191.

Popescu

, Popescu

, Dorobăț

, Rafilă

, Tănase

, Mikula

, et al. Oxa-48-carbapenemase-producing Klebsiella pneumoniae infections-the first cases diagnosed in romanian national institute of infectious diseases. Rev Rom Med Lab, 2017; 25:55–61.

192.

Pournaras

, Poulou

, Voulgari

, Vrioni

, Kristo

, Tsakris

. Detection of the new metallo-B-lactamase VIM-19 along with KPC-2, CMY-2 and CTX-M-15 in Klebsiella pneumoniae . J Antimicrob Chemother, 2010; 65:1604–1607.

193.

Prior

, Roche

, Lynch

, Kelly

, O'Rourke

, Crowley

. First identified case of VIM-producing carbapenemresistant Klebsiella pneumoniae in the Republic of Ireland associated with fatal outcome. Eurosurveillance, 2010; 15:19752.

194.

Projahn

, von Tippelskirch

, Semmler

, Guenther

, Alter

, Roesler

. Contamination of chicken meat with extended-spectrum beta-lactamase producing-Klebsiella pneumoniae and Escherichia coli during scalding and defeathering of broiler carcasses. Food Microbiol, 2019; 77:185–191.

195.

Prucek

, Ranc

, Kvítek

, Panáček

, Zbořil

, Kolář

. Reproducible discrimination between gram-positive and gram-negative bacteria using surface enhanced Raman spectroscopy with infrared excitation. Analyst, 2012; 137:2866–2870.

196.

Puspanadan

, Afsahhejri

, Loo

, Nillian

, Kuan

, Goh

, et al. Detection of Klebsiella pneumoniae in raw vegetables using most probable number-polymerase chain reaction (MPN-PCR). Int Food Res J, 2012; 19:1757–1762.

197.

Quan

, Li

, Chen

, Jiang

, Zhou

, Zhang

, et al. Prevalence of mcr-1 in Escherichia coli and Klebsiella pneumoniae recovered from bloodstream infections in China: A multicentre longitudinal study. Lancet Infect Dis, 2017; 17:400–410.

198.

Quiles

, Rocchetti

, Fehlberg

, Kusano

EJU

, Chebabo

, Pereira

RMG

, et al. Unusual association of NDM-1 with KPC-2 and armA among Brazilian Enterobacteriaceae isolates. Braz J Med Biol Res, 2015; 48:174–177.

199.

Ramirez

, Traglia

, Lin

, Tran

, Tolmasky

. Plasmid-mediated antibiotic resistance and virulence in Gram-negatives: The Klebsiella pneumoniae paradigm. Microbiol Spectr, 2014; 2:1–15.

200.

Regueiro

, Moranta

, Frank

, Larrarte

, Margareto

, March

, et al. Klebsiella pneumoniae subverts the activation of inflammatory responses in a NOD1-dependent manner. Cell Microbiol, 2011; 13:135–153.

201.

Rennie

, Anderson

, Wensley

, Albritton

, Mahony

. Klebsiella pneumoniae gastroenteritis masked by Clostridium perfringens . J Clin Microbiol, 1990; 28:216–219.

202.

Richardson

LA.

How Salmonella survives the macrophage's acid attack. PLoS Biol, 2015; 13:e1002117.

203.

Ridolfo

, Rimoldi

, Pagani

, Marino

, Piol

, Rimoldi

, et al. Diffusion and transmission of carbapenem-resistant Klebsiella pneumoniae in the medical and surgical wards of a university hospital in Milan, Italy. J Infect Public Health, 2016; 9:24–33.

204.

Robinson

, McComb

, Mulligan

, Dudani

, Krishnan

, Sad

. Type I interferon induces necroptosis in macrophages during infection with Salmonella enterica serovar Typhimurium TL-13. Nat Immunol, 2012; 13:954–962.

205.

Rodrigues

, Passet

, Rakotondrasoa

, Diallo

, Criscuolo

, Brisse

. Description of Klebsiella africanensis sp. nov., Klebsiella variicola subsp. tropicalensis subsp. nov. and Klebsiella variicola subsp. variicola subsp. nov. Res Microbiol, 2019; 170:165–170.

206.

Rolai

, Kempf

, Leangapichart

, Chabou

, Olaitan

, Le

, et al. Plasmid-mediated mcr-1 gene in colistin-resistant clinical isolates of Klebsiella pneumoniae in France and Laos. Antimicrob Agents Chemother, 2016; 60:6994–6995.

207.

Rose

, Schreier

. The effect of hospitalization and antibiotic therapy on the gram-negative fecal flora. Am J Med Sci, 1968; 255:228–236.

208.

Rosenblueth

, Martínez

, Silva

, Martínez-Romero

. Klebsiella variicola, a novel species with clinical and plant-associated isolates. Syst Appl Microbiol, 2004; 27:27–35.

209.

Ruckdeschel

, Roggenkamp

, Lafont

, Mangeat

, Heesemann

, Rouot

. Interaction of Yersinia enterocolitica with macrophages leads to macrophage cell death through apoptosis. Infect Immun, 1997; 65:4813–4821.

210.

Russo

, Olson

, MacDonald

, Beanan

, Davidsona

. Aerobactin, but not yersiniabactin, salmochelin, or enterobactin, enables the growth/survival of hypervirulent (hypermucoviscous) Klebsiella pneumoniae ex vivo and in vivo. Infect Immun, 2015; 83:3325–3333.

211.

Russo

, Olson

, MacDonald

, Metzger

, Maltese

, Drake

, et al. Aerobactin mediates virulence and accounts for increased siderophore production under iron-limiting conditions by hypervirulent (hypermucoviscous) Klebsiella pneumoniae . Infect Immun, 2014; 82:2356–2367.

212.

Sabota

, Hoppes

Jr ., Ziegler

, Dupont

, Mathewson

, Rutecki

GW.

A new variant of food poisoning: Enteroinvasive Klebsiella pneumoniae, and Escherichia coli, sepsis from a contaminated hamburger. Am J Gastroenterol, 1998; 93:118–119.

213.

Sahly

, Podschun

, Oelschlaeger

, Greiwe

, Parolis

, Hasty

, et al. Capsule impedes adhesion to and invasion of epithelial cells by Klebsiella pneumoniae . Infect Immun, 2000; 68:6744–6749.

214.

Saidani

, Messadi

, Mefteh

, Chaouechi

, Soudani

, Selmi

, Dâaloul-Jedidi

, Ben Chehida

, Mamlouk

, Jemli

, Madec

, Haenni

. Various Inc-type plasmids and lineages of Escherichia coli and Klebsiella pneumoniae spreading blaCTX-M-15,blaCTX-M-1 and mcr-1 genes in camels in Tunisia. J Glob Antimicrob Resist, 2019; 19:280–283.

215.

Saishu

, Ozaki

, Murase

. Ctx-m-type extended-spectrum β-lactamase-producing Klebsiella pneumoniae isolated from cases of bovine mastitis in Japan. J Vet Med Sci, 2014; 76:1153–1156.

216.

Santaniello

, Sansone

, Fioretti

, Menna

. Systematic review and meta-analysis of the occurrence of ESKAPE bacteria group in dogs, and the related zoonotic risk in animal-assisted therapy, and in animal-assisted activity in the health context. Int J Environ Res Public Health, 2020; 17:3278.

217.

Schønheyder

, Sanden

, Sørensen

. The prevalence of gentamicin resistance among clinical isolates of Enterobacteria in a Danish region. APMIS, 2000; 108:145–152.

218.

Schroll

, Barken

, Krogfelt

, Struve

. Role of type 1 and type 3 fimbriae in Klebsiella pneumoniae biofilm formation. BMC Microbiol, 2010; 10:179.

219.

Seidler

, Knittel

, Brown

. Potential pathogens in the environment: Cultural reactions and nucleic acid studies on Klebsiella pneumoniae from clinical and environmental sources. Appl Microbiol, 1975; 29:819–825.

220.

Sellick

, Russo

. Getting hypervirulent Klebsiella pneumoniae on the radar screen. Curr Opin Infect Dis, 2018; 31:341–346.

221.

Shahid

, Malik

, Adil

, Jahan

, Malik

. Comparison of beta-lactamase genes in clinical and food bacterial isolates in India. J Infect Dev Ctries, 2009; 3:593–598.

222.

Sheppard

, Stoesser

, Wilson

, Sebra

, Kasarskis

, Anson

, et al. Nested Russian doll-like genetic mobility drives rapid dissemination of the carbapenem resistance gene blaKPC. Antimicrob Agents Chemother, 2016; 60:3767–3778.

223.

Sheu

, Chang

, Lin

, Chen

, Hsueh

. Infections caused by carbapenem-resistant Enterobacteriaceae: An update on therapeutic options. Front Microbiol, 2019; 10:80.

224.

Shon

, Bajwa

RPS

, Russo

. Hypervirulent (hypermucoviscous) Klebsiella pneumoniae: A new and dangerous breed. Virulence, 2013; 4:107–118.

225.

Shooter

, Cooke

, Faiers

, Breaden

, O'Farrell

. Isolation of Escherichia coli, Pseudomonas aeruginosa, and Klebsiella from food in hospitals, canteens, and schools. Lancet, 1971; 2:390–392.

226.

Singh

, Kulshreshtha

. Preliminary examinations on the enterotoxigenicity of isolates of Klebsiella pneumoniae from seafoods. Int J Food Microbiol, 1992; 16:349–352.

227.

Sobirk

, Struve

, Jacobsson

. Primary Klebsiella pneumoniae liver abscess with metastatic spread to lung and eye, a North-European case report of an emerging syndrome. Open Microbiol J, 2010; 4:5–7.

228.

Soilleux

, Morand

, Arlet

, Scavizzi

, Labia

. Survey of Klebsiella pneumoniae producing extended-spectrum β-lactamases: Prevalence of TEM-3 and first identification of TEM-26 in France. Antimicrob Agents Chemother, 1996; 40:1027–1029.

229.

Soto

, LaMon

, Griffin

, Keirstead

, Beierschmitt

, Palmour

. Phenotypic and genotypic characterization of Klebsiella pneumoniae isolates recovered from nonhuman primates. J Wildl Dis, 2012; 48:603–611.

230.

Srikumar

, Kröger

, Hébrard

, Colgan

, Owen

, Sivasankaran

, et al. RNA-seq brings new insights to the intra-macrophage transcriptome of Salmonella typhimurium . PLoS Pathog, 2015; 11:e1005262.

231.

Srinivasan

, Rajamohan

. KpnEF, a new member of the Klebsiella pneumoniae cell envelope stress response regulon, is an SMR-type efflux pump involved in broad-spectrum antimicrobial resistance. Antimicrob Agents Chemother, 2013; 57:4449–4462.

232.

Srinivasan

, Singh

, Priyadarshi

, Chauhan

, Rajamohan

. Role of novel multidrug efflux pump involved in drug resistance in Klebsiella pneumoniae . PLoS One, 2014; 9:e96288.

233.

Stahlhut

, Schroll

, Harmsen

, Struve

, Krogfelt

. Screening for genes involved in Klebsiella pneumoniae biofilm formation using a fosmid library. FEMS Immunol Med Microbiol, 2010; 59:521–524.

234.

Stahlhut

, Tchesnokova

, Struve

, Weissman

, Chattopadhyay

, Yakovenko

, et al. Comparative structure-function analysis of mannose-specific FimH adhesins from Klebsiella pneumoniae and Escherichia coli . J Bacteriol, 2009; 191:6592–6601.

235.

Stiles

, Ng

. Enterobacteriaceae associated with meats and meat handling. Appl Environ Microbiol, 1981; 41:867–872.

236.

Stöckel

, Kirchhoff

, Neugebauer

, Rösch

, Popp

. The application of Raman spectroscopy for the detection and identification of microorganisms. J Raman Spectrosc, 2016; 47:89–109.

237.

Stoesser

, Mathers

, Moore

, Day

, Crook

. Colistin resistance gene mcr-1 and phnshp45 plasmid in human isolates of Escherichia coli and Klebsiella pneumoniae . Lancet Infect Dis, 2016; 16:285–286.

238.

Strauman

, Harper

, Harrington

, Boll

, Nataro

. Enteroaggregative Escherichia coli disrupts epithelial cell tight junctions. Infect Immun, 2010; 78:4958–4964.

239.

Struve

, Bojer

, Krogfelt

. Characterization of Klebsiella pneumoniae type 1 fimbriae by detection of phase variation during colonization and infection and impact on virulence. Infect Immun, 2008; 76:4055–4065.

240.

Struve

, Bojer

, Krogfelt

. Identification of a conserved chromosomal region encoding Klebsiella pneumoniae type 1 and type 3 fimbriae and assessment of the role of fimbriae in pathogenicity. Infect Immun, 2009; 77:5016–5024.

241.

, Zhou

, Luo

, et al. Genome-wide identification of genes regulated by RcsA, RcsB, and RcsAB phosphorelay regulators in Klebsiella pneumoniae NTUH-K2044. Microb Pathog, 2018; 123:36–41.

242.

Sudarwanto

, Akineden Ö, Odenthal

, Gross

, UsleberE. Extended-spectrum β-lactamase (ESBL)-producing Klebsiella pneumoniae in bulk tank milk from dairy farms in Indonesia. Foodborne Pathog Dis, 2015; 12:585.

243.

Sun

WSW

, Syu

, Ho

, Lin

, Tsai

, Wang

. SitA contributes to the virulence of Klebsiella pneumoniae in a mouse infection model. Microbes Infect, 2014; 16:161–170.

244.

Tan

JWH

, Wilksch

, Hocking

, Wang

, Srikhanta

, Tauschek

, et al. Positive autoregulation of mrkHI by the cyclic Di-GMP-dependent MrkH protein in the biofilm regulatory circuit of Klebsiella pneumoniae . J Bacteriol, 2015; 197:1659–1667.

245.

Tarkkanen

, Allen

, Westerlund

, Holthöfer

, Kuusela

, Risteli

, et al. Type V collagen as the target for type-3 fimbriae, enterobacterial adherence organelles. Mol Microbiol, 1990; 4:1353–1361.

246.

Tarkkanen

, Virkola

, Clegg

, Korhonen

. Binding of the type 3 fimbriae of Klebsiella pneumoniae to human endothelial and urinary bladder cells. Infect Immun, 1997; 65:1546–1549.

247.

Tekiner İH, Özpınar

Occurrence and characteristics of extended spectrum beta-lactamases-producing Enterobacteriaceae from foods of animal origin. Braz J Microbiol, 2016; 47:444–451.

248.

Tian

, Doi

, Shen

, Walsh

, Wang

, Zhang

, et al. Mcr-1-producing Klebsiella pneumoniae outbreak in china. Lancet Infect Dis, 2017; 17:577.

249.

Tominaga

Rapid detection of Klebsiella pneumoniae, Klebsiella oxytoca, Raoultella ornithinolytica and other related bacteria in food by lateral-flow test strip immunoassays. J Microbiol Methods, 2018; 147:43–49.

250.

Timofte

, Maciuc

, Evans

, Williams

, Wattret

, Fick

, et al. Detection and molecular characterization of Escherichia coli CTX-M-15 and Klebsiella pneumoniae SHV-12 β-lactamases from bovine mastitis isolates in the United Kingdom. Antimicrob Agents Chemother, 2014; 58:789–794.

251.

Tomás

, Lery

, Regueiro

, Pérez-Gutiérrez

, Martínez

, Moranta

, et al. Functional genomic screen identifies Klebsiella pneumoniae factors implicated in blocking nuclear factor κB (NF-κB) signaling. J Biol Chem, 2015; 290:16678–16697.

252.

Turner

, Cahill

, Dilello

, Gutel

, Brunson

, Albertí

, et al. Porin loss impacts the host inflammatory response to outer membrane vesicles of Klebsiella pneumoniae . Antimicrob Agents Chemother, 2016; 60:1360–1369.

253.

Turner

, Hanage

, Fraser

, Connor

, Spratt

, Mazars

, et al. Assessing the reliability of eBURST using simulated populations with known ancestry. BMC Microbiol, 2007; 7:30.

254.

Tzouvelekis

, Markogiannakis

, Psichogiou

, Tassios

, Daikos

. Carbapenemases in Klebsiella pneumoniae and other Enterobacteriaceae: An evolving crisis of global dimensions. Clin Microbiol Rev, 2012; 25:682–707.

255.

Tzouvelekis

, Tzelepi

, Tassios

, Legakis

. CTX-M-type β-lactamases: An emerging group of extended-spectrum enzymes. Int J Antimicrob Agents, 2000; 14:137–142.

256.

Upadhyay

, Hussain

, Ingti

, et al. Detection of a new class C β-lactamase (CMY-139) in Klebsiella pneumoniae of food origin from India. J Glob Antimicrob Resist, 2017; 8:46–47.

257.

Vila

, Cassata

, Pagella

, Amadio

, Yeh

K-M

, Chang

F-Y

, et al. Appearance of Klebsiella pneumoniae liver abscess syndrome in Argentina: Case report and review of molecular mechanisms of pathogenesis. Open Microbiol J, 2011; 5:107–113.

258.

Voor in't Holt

, Severin

, Goessens

, Te Witt

, Vos

. Instant typing is essential to detect transmission of extended-spectrum beta-lactamase-producing Klebsiella species. PLoS One, 2015; 28;10:e0136135.

259.

Wang

, Zeng

, Huang

, et al. Evolution and comparative genomics of F33:A-:B-plasmids carrying blaCTX-M-55 or blaCTX-M-65 in Escherichia coli and Klebsiella pneumoniae isolated from animals, food products, and humans in China. mSphere, 2018; 3:e00137-18.

260.

Wang

, Wang

, Ouyang

, Jin

, Wang

, Zhang

, Jin

, Chen

, Wang

, Zhang

, Cao

, Xie

, Liao

, Gu

, Yang

, Liu

, Ma

, Jin

, Zhang

, Man

, Li

, Pei

, Xu

, Jin

, Ji

, Wang

. 2018. Phenotypic and genotypic characterization of carbapenem-resistant Enterobacteriaceae: Data from a longitudinal large-scale CRE study in China (2012–2016). Clin Infect Dis, 2018; 67:S196–S205.

261.

Wang

, Liu

, Zhang

, et al. The prevalence of colistin resistance in Escherichia coli and Klebsiella pneumoniae isolated from food animals in China: Coexistence of mcr-1 and blaNDM with low fitness cost. Int J Antimicrob Agents, 2018; 51:739–744.

262.

Webster

, Daigneault

, Bewley

, Preston

, Marriott

, Walmsley

, et al. Distinct cell death programs in monocytes regulate innate responses following challenge with common causes of invasive bacterial disease. J Immunol, 2010; 185:2968–2979.

263.

Wei

C-Y

, Xing

Y-X

, Lin

, Yang

L-T

, Li

Y-R

, Hu

C-J

. [Growth-promoting effect of inoculating Klebsiella variicola DX120E on different sugarcane cultivars]. Ying Yong Sheng Tai Xue Bao, 2014; 25:2085–2092 (Article in Chinese).

264.

Whitehouse

, Keirstead

, Taylor

, Reinhardt

, Beierschmitt

. Prevalence of hypermucoid Klebsiella pneumoniae among wild-caught and captive vervet monkeys (Chlorocebus aethiops sabaeus) on the island of St. Kitts. J Wildl Dis, 2010; 46:971–976.

265.

Williams

, Lambert

, Brown

, Jones

. The role of the O and K antigens in determining the resistance of Klebsiella aerogenes to serum killing and phagocytosis. J Gen Microbiol, 1983; 129:2181–2191.

266.

Willingham

, Allen

, Bergstralh

, Brickey

, Huang

MT-H

, Taxman

, et al. NLRP3 (NALP3, cryopyrin) facilitates in vivo caspase-1 activation, necrosis, and HMGB1 release via inflammasome-dependent and -independent pathways. J Immunol, 2009; 183:2008–2015.

267.

Witkowska

, Korsak

, Kowalska

, Księżopolska-Gocalska

, Niedziółka-Jönsson

, Roźniecka

, Michałowicz

, Albrycht

, Podrażka

, Hołyst

, Waluk

, Kamińska

. Surface-enhanced Raman spectroscopy introduced into the International Standard Organization (ISO) regulations as an alternative method for detection and identification of pathogens in the food industry. Anal Bioanal Chem, 2017; 409:1555–1567.

268.

C-C

, Huang

Y-J

, Fung

C-P

, Peng

H-L

. Regulation of the Klebsiella pneumoniae Kpc fimbriae by the site-specific recombinase KpcI. Microbiology, 2010; 156:1983–1992.

269.

, Wang

, Liu

, Wang

, Lu

, et al. Characterization of antimicrobial resistance in Klebsiella species isolated from chicken broilers. Int J Food Microbiol, 2016; 232:95–102.

270.

, Li

, Yan

, Tsao

, Liao

, Tsai

, et al. Genome sequencing and comparative analysis of Klebsiella pneumoniae NTUH-K2044, a strain causing liver abscess and meningitis. J Bacteriol, 2009; 191:4492–4501.

271.

, Chen

, Lin

, Hsieh

, Wang

. Cellobiose-specific phosphotransferase system of Klebsiella pneumoniae and its importance in biofilm formation and virulence. Infect Immun, 2012; 80:2464–2472.

272.

, Lin

, Hsieh

, Yang

, Wang

. Isolation of genes involved in biofilm formation of a Klebsiella pneumoniae strain causing pyogenic liver abscess. PLoS One, 2011; 6:e23500.

273.

Wyres

, Hawkey

, Hetland

MAK

, Fostervold

, Wick

, Judd

, Hamidian

, Howden

, Löhr

, Holt

. Emergence and rapid global dissemination of CTX-M-15-associated Klebsiella pneumoniae strain ST307. J Antimicrob Chemother, 2019; 74:577–581.

274.

Wyres

, Holt

. Klebsiella pneumoniae population genomics and antimicrobial-resistant clones. Trends Microbiol, 2016; 24:944–956.

275.

Wyres

, Holt

. Klebsiella pneumoniae as a key trafficker of drug resistance genes from environmental to clinically important bacteria. Curr Opin Microbiol, 2018; 45:131–139.

276.

Wyres

, Lam

MMC

, Holt

. Population genomics of Klebsiella pneumoniae . Nat Rev Microbiol, 2020; 18:344–359.

277.

Wyres

, Wick

, Judd

, Froumine

, Tokolyi

, Gorrie

, Lam

, Duchêne

, Jenney

, Holt

. Distinct evolutionary dynamics of horizontal gene transfer in drug resistant and virulent clones of Klebsiella pneumoniae . PLoS Genet, 2019; 15:e1008114.

278.

Xavier

, Lammens

, Ruhal

, Kumar-Singh

, Butaye

, Goossens

, et al. Identification of a novel plasmid-mediated colistin-resistance gene, mcr-2, in Escherichia coli, Belgium, June 2016. Eurosurveillance, 2016; 21 [Epub ahead of print]; DOI: 10.2807/1560-7917.ES.2016.21.27.30280.

279.

, Mackay

, Sommers

. Draft genomic sequence of multidrug-resistant Klebsiella pneumoniae B8S35, isolated from retail chicken skin. Microbiol Resour Announc, 2019; 8:e00502-19.

280.

Xuan Nguyen

, Sarter

, Hai Nguyen

, Daniel

. Detection of molecular changes induced by antibiotics in Escherichia coli using vibrational spectroscopy. Spectrochim Acta A Mol Biomol Spectrosc, 2017; 183:395–401.

281.

Yan

, Wang

, Ning

, Wang

, Fang

, Wang

. Detection and contamination about Klebsiella pneumoniae in the infant formula pouder. China Dairy Ind, 2011; 39:56–58.

282.

Yang

, Shen

, Zheng

, Liu

, El-Sayed Ahmed

, Zhao

, Liao

, Shi

, Guo

, Zhong

, Xu

, Tian

. Plasmid-mediated colistin resistance gene mcr-1 in Escherichia coli and Klebsiella pneumoniae isolated from market retail fruits in Guangzhou, China. Infect Drug Resist, 2019; 12:385–389.

283.

Yaseen

, Sun

, Cheng

. Raman imaging for food quality and safety evaluation: Fundamentals and applications. Trends Food Sci Technol, 2017; 62:177–189.

284.

W-L

, Ko

W-C

, Cheng

K-C

, Lee

C-C

, Lai

C-C

, Chuang

Y-C

, et al. Comparison of prevalence of virulence factors for Klebsiella pneumoniae liver abscesses between isolates with capsular K1/K2 and non-K1/K2 serotypes. Diagn Microbiol Infect Dis, 1991; 62:1–6.

285.

Yin

, Hui

, Shen

, Liu

, Wang

, Shen

, et al. Novel plasmid-mediated colistin resistance gene mcr-3 in Escherichia coli . mBio, 2017; 8:e00543-17.

286.

Zhang

, Han

, Liu

, Luo

, Lu

, He

. Occurrence of selected zoonotic fecal pathogens and first molecular identification of Hafnia paralvei in wild taihangshan macaques (Macaca mulatta tcheliensis) in China. Biomed Res Int, 2019; 2019:2494913.

287.

Zhang

, Yang

, Ye

, Wu

, Zhang

, Huang

. Phenotypic and genotypic characterization of Klebsiella pneumoniae isolated from retail foods in China. Front Microbiol, 2018; 9:289.

288.

Zhang

, Zhu

, Wang

, Liu

, Li

, Chen

, Luan

, Zhang

, Schwarz

, Liu

. Characterization of a multidrug-resistant porcine Klebsiella pneumoniae sequence type 11 strain coharboring blaKPC-2 and fosA3 on two novel hybrid plasmids. mSphere, 2019; 4:e00590-19.

289.

Zhang

, Li

, Wang

, Yue

, Li

, Liu

, et al. Outbreak of ndm-1-producing Klebsiella pneumoniae causing neonatal infection in a teaching hospital in mainland China. Antimicrob Agents Chemother, 2015; 59:4349–4351.

290.

Zhao

, Yu

, Lü

, Mcnally

, Zong

. Incp plasmid carrying colistin resistance gene mcr-1 in Klebsiella pneumoniae from hospital sewage. Antimicrob Agents Chemother, 2017; 61:e02229-16.

291.

Zhao

, Li

, Xu

. Detection of foodborne pathogens by surface enhanced Raman spectroscopy. Front Microbiol, 2018; 9:1–13.

292.

Zheng

, Zhang

, Guo

, Feng

, Liu

, Zhang

, et al. Outbreak of plasmid-mediated NDM-1-producing Klebsiella pneumoniae ST105 among neonatal patients in Yunnan, China. Ann Clin Microbiol Antimicrob, 2016; 15:10.

293.

Zhu

, Ding

, Xu

, Zhu

, Yang

, Zhang

, et al. Klebsiella pneumoniae: Development of carbapenem resistance due to acquisition of blaNDM-1 during antimicrobial therapy in twin infants with pneumonia. Front Microbiol, 2015; 6:1399.

294.

Zurfluh

, Poirel

, Nordmann

, Klumpp

, Stephan

. First detection of Klebsiella variicola producing OXA-181 carbapenemase in fresh vegetable imported from Asia to Switzerland. Antimicrob Resist Infect Control, 2015; 4:38.