Investigation of the In Vitro Effectiveness of Aztreonam/Avibactam,Colistin/Apramycin,and Meropenem/Apramycin Combinations Against Carbapenemase-Producing,Extensively Drug-Resistant Klebsiella pneumoniae Strains

Abstract

This study aimed at investigating the in vitro effectiveness of aztreonam/avibactam, colistin/avibactam, colistin/apramycin, and meropenem/apramycin combinations against carbapenemase-producing, extensively drug-resistant (XDR) Klebsiella pneumoniae strains. This study evaluated 38 carbapenem-resistant, carbapenemase-producing, and XDR K. pneumoniae strains. The checkerboard method was used to examine the efficacy of aztreonam/avibactam, and meropenem/apramycin combinations in all strains and the colistin/apramycin combination in colistin-resistant strains (n = 26). It was found that when used alone, aztreonam and avibactam had high minimum inhibitory concentration values in all strains and that all strains were resistant to aztreonam. Nevertheless, the aztreonam/avibactam combination was found to have a synergistic effect against all strains. Apramycin alone was effective against 30 K. pneumoniae strains (79%); however, 8 strains (21%) were found to be resistant. In the synergy testing of 26 colistin-resistant strains with the checkerboard method, the colistin/apramycin combination was found to have a synergistic effect against 4 strains (15.3%), an antagonistic effect against 8 strains (30.7%), and an additive effect against 14 strains (54%). By comparison, the meropenem/apramycin combination had a synergistic effect against 20 strains (52%) and an additive effect against 12 strains (31%). The aztreonam/avibactam combination showed a high in vitro synergistic effect on carbapenemase-producing and XDR K. pneumoniae strains, such as Metallo-β-lactamase, and provided good prospects for the successful treatment. The meropenem/apramycin combination was also highly synergistic. The synergistic effects were low for the colistin/apramycin combination that was tested on colistin-resistant strains. However, it is promising that apramycin has low minimal inhibitory concentration values.

Introduction

The emergence of carbapenemase-producing Enterobacteriaceae (CPE) strains has given rise to various infections that are characterized by high rates of mortality, which have, in turn, prompted the intense administration of various antibiotics for attempts at treatment.^1,2 These carbapenem-resistant strains, which were initially viewed as health care-associated infections, began to spread as infections throughout communities, as well.^3,4 The spread of carbapenem-resistant strains, particularly epidemic and sporadic cases of Klebsiella pneumoniae across the globe, have brought about studies to investigate resistance mechanisms and new treatment possibilities for these bacteria.⁵ The existence of additional resistance mechanisms in CPE strains has given rise to phenotypes that are resistant to antibiotics that are not part of the beta-lactam family (e.g., fluoroquinolone and aminoglycoside-resistant strains).^6,7 Other therapeutic options (e.g., colistin, tigecycline, fosfomycin, etc.) are insufficient due to concerns about their effectiveness and toxicity profiles.^8,9

Although the administration of appropriate treatments for CPE infections is an evolving problem, no clear data are available on this topic. To date, there are a limited number of randomized and controlled studies that have investigated the anti-microbial treatment options for CPE infections. For this reason, most of the available data are derived from case reports, case series, and small-scale retrospective studies with a series of natural limitations.^10,11

Clinicians have had to re-assess the use of agents, such as polymyxin, Fosfomycin, and amino-glycosides, that are rarely, if ever, used due to concerns about their effectiveness and/or toxicity. All of these problems have made it necessary to perform original research on new agents, for use in treatment and combination therapies. It is believed that combination therapies can increase the potential for successful and empirical anti-microbial therapies. Trials of combination therapies are often sought after, because the agents involved in these combinations produce synergistic effects, because they reduce possible resistances to therapy that results from the inhibition of bacteria, and because they limit the selection and reproduction of the resistant populations in strains that are hetero-resistant to the antibiotic agents used in therapies.^12,13

Because colistin is an agent that is likely to have a lower minimal inhibitor concentration despite its toxicity, its use is preferred during monotherapy or in combination with other agents for the treatment of K. pneumoniae infections. Although amino-glycosides are another group of antibiotics that are used as alternatives during therapy, it is difficult for them to produce effective therapies due to their high toxicities and the additional resistance mechanisms to which they are subjected.

Apramycin, an amino-glycoside used in veterinary medicine, differs from other amino-glycosides because of its molecular structure. Due to this unique molecular structure, it has less nephrotoxicity and autotoxicity than other amino-glycosides, such as amikacin and gentamicin. At the same time, since the characteristics of amino-glycosides are less affected by bacterial ribosomal mutations and modifying enzymes that are widely produced by bacteria, certain researchers have suggested that apramycin can prove effective against amino-glycoside-resistant strains.¹⁴ Thus, because apramycin can provide effective and less toxic therapy against multi-drug resistant bacteria, it is worth investigating its viability in monotherapy and combination therapies.

Another alternative that is frequently used in the treatment of β-lactamase producing strains is combined use of an effective β-lactamase inhibitor with a β-lactam antibiotic. Avibactam, which is a β-lactamase inhibitor, can inhibit all carbapenemase enzymes (including OXA-48 and KPC), except for Metallo-β-lactamase (MBL). The ceftazidime/avibactam combination has been approved in Europe and the United States and has seen wide use in surveillance studies. Because it is possible for the ceftazidime/avibactam combination to effectively treat strains that produce carbapenemase and extended-spectrum beta-lactamases (ESBL) enzymes, such as OXA-48 and KPC,^15,16 a combination of avibactam and other agents, such as aztreonam, could prove effective against MBL or as a treatment alternative for strains that produce MBL.¹⁷

This study aimed at investigating the in vitro effects of aztreonam/avibactam, colistin/apramycin, and meropenem/apramycin combinations against carbapenemase-producing and extensively drug-resistant (XDR) K. pneumoniae strains. It also sought to obtain data about their usage possibilities in the treatment of infectious diseases caused by these types of resistant micro-organisms.

Materials and Methods

Selection of clinical isolates

This study used K. pneumoniae strains isolated from rectal swabs and several other samples from the culture collection unit at Sakarya University's Training and Research Hospital in the Medical Microbiology Laboratory.¹⁸ Routine identifications of the strains and antibiotic susceptibility tests were performed by using the automated VITEK 2^® system (bioMerieux, France). Subsequently, the VITEK^® MS system (bioMerieux) was used to confirm the identifications of the strains. Thirty-eight carbapenemase-producing XDR strains were included in the study. Table 1 shows the clinical sample and enzyme types of strains included in the study.

Table 1.

Sample Characteristics and Carbapenemase Content of Klebsiella pneumoniae Isolates

Strain no.	Sample types	Clinics	Enzyme types
1	Blood	ICU	OXA-48
2	Blood	ICU	NDM-1
3	Blood	ICU	NDM-1
4	Rectal swab	ICU	NDM-1 + OXA-48
5	Rectal swab	ICU	NDM-1
6	Blood	ICU	NDM-1 + OXA-48
7	Rectal swab	ICU	NDM-1
8	Rectal swab	ICU	NDM-1
9	Blood	ICU	KPC
10	Rectal swab	ICU	NDM-1 + OXA-48
11	Rectal swab	ICU	NDM-1
12	Blood	Infectious diseases	OXA-48
13	Wound	Internal medicine	OXA-48
14	Urine	ICU	NDM-1
15	Wound	ICU	NDM-1
16	Urine	Internal medicine	NDM-1
17	Tracheal aspirate	ICU	OXA-48
18	Tracheal aspirate	ICU	NDM-1
19	Blood	ICU	NDM-1
20	Wound	General surgery	NDM-1
21	Rectal swab	ICU	NDM-1 + OXA-48
22	Blood	Pediatry	OXA-48
23	Urine	General surgery	OXA-48
24	Urine	Internal medicine	NDM-1
25	Rectal swab	ICU	OXA-48
26	Rectal swab	ICU	KPC
27	Wound	Infectious diseases	OXA-48
28	Blood	ICU	OXA-48
29	Rectal swab	ICU	OXA-48
30	Rectal swab	ICU	OXA-48
31	Rectal swab	ICU	OXA-48
32	Rectal swab	ICU	KPC
33	Rectal swab	ICU	KPC
34	Rectal swab	ICU	KPC
35	Rectal swab	ICU	KPC
36	Rectal swab	ICU	KPC
37	Rectal swab	ICU	KPC
38	Rectal swab	ICU	KPC

ICU, intensive care unit.

Identification of carbapenemase production

The Modified Hodge Test and Rapidec^® Carba NP (bioMerieux), which are phenotypic methods, were tested as described in the Clinical and Laboratory Standards Institute's (CLSI) guidelines and according to the manufacturer's recommendations.¹⁹ The Xpert^® Carba-R kit (bla_KPC, bla_NDM, bla_VIM, bla_OXA-48, and bla_IMP) and GeneXpert device (Cepheid) was then used to detect genotypic carbapenemase genes (Table 1).

Antibiotic susceptibility testing by broth micro-dilution method

The antibiotics used in this study, colistin, apramycin, aztreonam, avibactam, and meropenem, were procured in powdered form (Cayman Chemical, MI). The minimal inhibitory concentration (MIC) values of the strains were determined according to the CLSI's micro-dilution procedure.²⁰ Escherichia coli ATCC 25922 was used as the BMD method's quality-control strain.

The antibiotic concentrations that were used were selected to include the CLSI's and the European Committee on Antimicrobial Susceptibility Testing (EUCAST)'s breakpoints.²¹ The limit values for E. coli were used in the Centers for Disease Control and Prevention's (CDC) National Antibiotic Resistance Monitoring System report, because the apramycin clinical limit values for the Klebsiella species were not included in these guidelines.²²

Synergy testing with the checkerboard method

Synergy testing was conducted with the checkerboard micro-dilution method.²³ By using this method, the MIC values for the aztreonam/avibactam, colistin/apramycin, and meropenem/apramycin combinations were determined and the fractional inhibitory concentration (FIC) index was calculated. The synergistic effect was defined as an FIC index of ≤0.5, the additive or indifferent effect was defined as 0.5 < FIC <4. and the antagonistic effect was defined as an FIC index of >4.²⁴

Results

Table 2 shows the susceptibility rates of the investigated K. pneumoniae strains to the antibiotics used in the study determined with the BMD technique. The results of the evaluation with the checkerboard FIC index of the antibiotic combinations tested against the strains investigated are set out in Table 3.

Table 2.

Susceptibility Rates of the Investigated Klebsiella pneumoniae Strains to the Antibiotics Used in the Study Determined with the Broth Micro-Dilution Method

Antibiotics	Susceptibility^a
Susceptible	Non-susceptible
n	%	n	%
Colistin	12	32	26	68
Meropenem	4	11	34	89
Aztreonam	0	0	38	100
Apramycin	MIC value and susceptibility ^* ^b

	Susceptible	Non-susceptible
	2 μg/mL	4 μg/mL	8 μg/mL	>256 μg/mL
	n	%	n	%	n	%	n	%
6	16	23	60	1	3	8	21

CLSI, Clinical and Laboratory Standards Institute; EUCAST, European Committee on Antimicrobial Susceptibility Testing; MIC, minimal inhibitory concentration.

EUCAST, Clinical breakpoints-bacteria (v 8.1). CLSI, M100 ED28.

National Antibiotic Resistance Monitoring System (NARMS) Working Group. Annual report. (2001).

Table 3.

Evaluation of the Antibiotic Combinations According to Fractional Inhibitory Concentration Index Values of the Checkerboard Method

Combinations (n = 38)	Synergistic	Additive	Antagonistic	Could not be assessed
n	%	n	%	n	%	n
Aztreonam/avibactam	38	100	—	—	—	—	—
Meropenem/apramycin	20	52.6	12	31.5	—	—	6^a
Colistin/apramycin	4	15.3	14	53.8	8	30.1	12^a

FIC, fractional inhibitory concentration.

Because the MIC values were very high in this combination and singly (>64 μg/mL), FIC index could not be calculated.

It could not be tested, because colistin has very low MIC values.

Aztreonam and avibactam

All of the strains were resistant to aztreonam when used alone, according to the BMD method [MIC values of all strains >64 μg/mL, (Table 2)]. Based on MIC values of the combination and CLSI's breakpoints of aztreonam, all of the strains (38/38; 100%) were susceptible (MIC values of all strains ≤4 μg/mL). According to the EUCAST's breakpoints, it was determined that 36 out of 38 (94.7%) of the strains were susceptible to aztreonam when used combined (MIC values of 36 strains ≤1 μg/mL); the remaining 2 strains were susceptible to increased dosage exposure (MIC values of two strains = 2 μg/mL). These assessments were performed independently from the MIC values in the avibactam combination. Based on the combination of the MIC values in all strains, the MIC values for avibactam were demonstrated to range from 0.5 to 8 μg/mL.

In cases where the avibactam concentration in the aztreonam/avibactam combination was considered to be fixed at 4 μg/mL (e.g., the fixed avibactam concentration in the ceftazidime/avibactam combination defined in manuals), the study showed that 37 out of 38 (97.3%) of the strains were susceptible to aztreonam, and 1 strain was intermediate susceptible according to the CLSI's aztreonam breakpoints. According to the EUCAST's threshold values, 31 out of 38 (81.6%) of the strains were susceptible to aztreonam, 2 (5.3%) were susceptible to higher doses, and 5 (13.1%) were in the MIC-resistance range. These 7 strains (2 intermediates and 5 resistant strains) were found to be KPC positive by Xpert Carba-R kit.

Colistin and apramycin

For the evaluations using the BMD method, 12 out of 38 (31%) K. pneumoniae strains were found to be susceptible to colistin and to have lower MIC values. As such, these strains were not included in the colistin/apramycin synergy test. Resistance to colistin was detected in 26 (69%) of the strains. When the efficacy of the colistin/apramycin combination was evaluated according to FIC index values for 26 colistin-resistant strains, 4 strains (15.3%) had a synergistic effect, 14 strains (54%) had an additive effect, and 8 strains (30.7%) had an antagonistic effect (Table 3).

Apramycin was effective in 30 out of 38 (79%) of the strains when used alone. The other 8 strains (21%) were found to be resistant. Seven of the apramycin-resistant strains were KPC, and one of them was NDM-1. Eleven out of 38 (28.9%) strains tested were susceptible to amikacin, and 4 (10.5%) were susceptible to gentamicin.

Nineteen (73%) of the colistin-resistant strains were susceptible to apramycin. In 13 out of these 19 strains, the combination MIC values of colistin decreased by 2 times and more; no decrease in MIC values of 6 strains was observed. As a result, it was found that all 4 strains detected synergistic effects for colistin/apramycin combination and were within these 13 strains.

Meropenem and apramycin

According to MIC values that were measured by the BMD method, only 4 (10.5%) of the strains were susceptible to meropenem. In the meropenem/apramycin combination, 20 strains (52%) had a synergistic effect and 12 (31%) had an additive effect. Although none of the strains had an antagonistic effect, synergy was unable to be evaluated because the MIC values for the combinations of the other six strains were high (>128 μg/mL) (Table 3). On the other hand, for the 26 colistin-resistant strains, 11 (42%) had a synergistic effect and 9 (34.5%) had an additive effect. Based on the MIC values for meropenem in the combination, 13 strains (34%) reached MIC values that illustrated a susceptibility to meropenem. The MIC value for apramycin in the combination was found to be at the lower range limit (0.5–2 μg/mL) against 31 (81.5%) of the strains. In addition, 26 (76.4%) of 34 meropenem-resistant strains were susceptible to apramycin.

Discussion

Carbapenem-resistant Enterobacteriaceae strains can cause various serious infections of the respiratory tract and urinary system, as well as bacteremia and intra-abdominal infections, which are resistant to almost all antibiotics that are currently available. In Zhang et al.'s study (2015), which investigated 664 carbapenem-resistant Enterobacteriaceae infections that had been reported in 25 Chinese hospitals, they observed that the K. pneumoniae (73.3%), E. coli (16.6%), and Enterobacter cloacae (7.1%) species were causative in most of these cases. This study found a general mortality rate of 33.5% for these bacterial infections.²⁵ Consistent with the results observed in previous research, the results of this study indicated that the species of K. pneumoniae were dominant among carbapenem-resistant strains. Among carbapenem-resistant Enterobacteriaceae species, our study also investigated carbapenemase-producing and highly drug-resistant K. pneumoniae strains.

Several studies, including global surveillance programs, have reported that aztreonam/avibactam therapy, a combination that can act as an alternative in the treatment of CPE strains, is very effective in treating these strains. It has also been noted that in addition to its effectiveness against serine β-lactamases, such as OXA-48 and KPC, it can also successfully treat MBL-producing strains, such as NDM-1.^26,27 In Biedenbach et al.'s global surveillance study, they asserted that the aztreonam/avibactam combination had a high in vitro effectiveness against CPE strains, such as MBL.²⁸

Our study also demonstrated that the aztreonam/avibactam combination was effective and synergistic against almost every strain. This combination is not only effective against carbapenemase-producing strains in the KPC and OXA-48 group but can also produce successful results against MBL (NDM-1) producing strains and dual-carbapenemase-producing strains (four strains that produce NDM-1+OXA-48). This versatility is vital to the combination's ability to serve as an alternative treatment option.

Existing research has observed that the ceftazidime/avibactam combination, another combination of avibactam that has been developed to treat resistant strains, is ineffective against MBL-producing strains.^29,30 Unlike ceftazidime, aztreonam is resistant to MBL enzymes. Even though avibactam does not inhibit MBL enzymes in the aztreonam/avibactam combination, it does inhibit other co-existing and probable β-lactamases that hydrolyze aztreonam (e.g., the β-lactamases of ESBL, AmpC, OXA-48, KPC, etc.). This may explain the activity/synergistic feature of this combination against MBL-producing strains. Therefore, if used for future treatment, further research on MBL production in carbapenem-resistant strains may provide valuable insights. Moreover, there may be preference for this combination during initial treatment phases against strains that produce MBL.

The clinical phase III experiments on the aztreonam/avibactam combination are currently ongoing; nonetheless, it is highly probable that the combination will soon be introduced for treatment, for although these antibiotics have yet to prove effective in their combined form, both drugs are already used in clinical practice (in the form of either the ceftazidime/avibactam combination or aztreonam as a single drug). It is also worth noting that international manuals do not yet include the current clinical threshold values for the aztreonam/avibactam combination.

The in vitro activities of the aztreonam/avibactam combination against MBL-producing strains are currently considered successful, whereas the results of Lohans et al.'s recent study suggest that MBL enzymes, such as NDM-1 and VIM, display hydrolytic activity against these two antibiotics that present a source for concern. Lohans et al. argued that their findings should be taken into consideration, because this may be the cause for aztreonam/avibactam's potential resistance in clinical practice. Regardless, this subject remains worthy of discussion, as other findings in existing research on enzymes have established that these agents have no hydrolytic activity against monobactams.³¹

Colistin susceptibility testing has undergone various changes over the years, and its use remains a matter of debate. Manuals recommend using colistin in the form of colistin sulfate during susceptibility tests. As a common point, the CLSI's and EUCAST's agar-diffusion methods are not currently recommended, whereas using the BMD method to determine the MIC value is advised.

In this study, although 6 of the strains that the VITEK 2 automated system detected were determined to be resistant by using the BMD method, 2 of the resistant strains were found to be susceptible to colistin. In light of these dissimilarities, further research should be conducted to examine the differences between colistin susceptibility tests by automated systems and MIC values using the BMD method. The BMD method was used as the baseline for testing the colistin susceptibility of the strains used in the colistin/apramycin combination. Therefore, in using the BMD method, 26 of the XDR CPE strains were defined to be colistin resistant and were included in the colistin/apramycin combination trial.

A limited number of studies have investigated colistin/amino-glycoside combinations in an in vitro environment. In one of these studies, wherein Chitnis et al. assessed the synergy of amikacin and gentamicin individually in a combination with colistin by using the checkerboard method, they found that 46 gram-negative strains (10 of them were Klebsiella spp.) had synergistic effects of 19.5% and 10.8%, respectively. The study also determined that the other strains had additive effects but no antagonistic effects.³² In another study that evaluated Acinetobacter, Pseudomonas, and Klebsiella bacteria and their resistance to several antibiotics by using various synergy methods, the polymyxin B/amikacin combination was used against only one K. pneumoniae strain, producing results that were synergistic with the time-kill method and additive when using the checkerboard technique.³³

Literature reviews on colistin/apramycin combinations have revealed that no clinical or laboratory research is currently available. As there are a limited number of agents that are effective against resistant strains, apramycin, which has gained credence in in vitro studies, has yet to be authorized for use in human medicine. This study, which is one of the first-ever laboratory studies to observe the use of apramycin in combination with colistin, investigated whether the antibiotic has a synergistic effect against carbapenemase-producing and colistin-resistant K. pneumoniae strains. In this combination, a synergistic effect was detected in only four strains (15.3%), whereas other strains had antagonistic (30.7%) or additive (54%) effects. Consistent with the results observed about other aminoglycosides in previous research, the frequency with which the colistin/apramycin combination produced a synergistic effect was found to be low. Unlike several of the aforementioned studies, which indicated a similar rate wherein colistin/amino-glycoside produced an additive effect, this study determined that this combination produced an antagonistic effect at a significant rate.

The EUCAST's and CLSI's guidelines do not include the MIC threshold values for apramycin, restricting any optimal evaluation of its efficacy. Notwithstanding other studies on apramycin, this specific feature has led several investigations of its effectiveness to compare its MIC values with other amino-glycosides (e.g., amikacin, gentamicin or tobramycin) or to use the threshold values that the national working group declared for E. coli in association with the Global Antimicrobial Resistance Monitoring System (GLASS) in their final 2001 report. Several studies have used these threshold values to investigate apramycin's effectiveness against various bacteria groups. Recent research has also highlighted that apramycin can be effective against carbapenem-resistant strains. In Smith and Kirby's investigation of Enterobacteriaceae strains that were susceptible and resistant to carbapenem, they determined apramycin's effectiveness in all strains, including carbapenem-resistant strains, to be at rates of 78% and 70.8%, respectively. This study found that the effectiveness of other amino-glycosides against carbapenem-resistant strains was 47.2% for gentamicin, 34.7% for tobramycin, and 65.3% for amikacin, indicating significant differences between amino-glycosides that did not include amikacin.³⁴ In a recent study, Galani et al. reported that apramycin was effective against carbapenem-resistant Enterobacteriaceae (n = 411) and carbapenem-resistant Acinetobacter baumannii (n = 594) strains, with effectiveness rates of 94.9% and 88.6%, respectively.³⁵ In our study, 79% of the strains produced MIC values that were at susceptibility limits and apramycin proved effective in 73% of the colistin-resistant strains. These susceptibility rates were substantially higher than those recorded for other amino-glycosides (e.g., amikacin: 28.9%; gentamicin: 10.5%). This finding suggests that in addition to apramycin's extensive drug resistance, it is also effective against most colistin-resistant strains on its own. Further comprehensive in vivo and in vitro studies should be conducted to provide further support for these findings.

Our study also found that most KPC-producing colistin-resistant strains (7/8; 87.5%) were resistant to apramycin. Apramycin's MIC value was >512 μg/mL when used alone, whereas it decreased to 2 μg/mL when used in a combination. However, because the MIC values of the colistin combinations against these strains increased by two- to eight-fold depending on the FIC index, no synergistic effect was detected in the combination of the colistin/apramycin combination in question. Since no synergistic effect was detected in the majority of the investigated strains, and the amino-glycoside/colistin combination was associated with a risk of toxicity, we can conclude that the use of this combination is inappropriate.

In another in vitro study that examined meropenem/amino-glycoside combinations using the time-kill method, Kulengowski et al. demonstrated that the meropenem/amikacin combination had a synergistic and bactericidal effect against CPE. They reported that the four strains used in their study were susceptible to amikacin.³⁶ In another study that used the checkerboard method, the plazomicin/meropenem combination was determined to have a synergistic effect at a rate of 20% but displayed no antagonistic effect.³⁷

As one of the first studies to investigate the meropenem/apramycin combination, this study found that 52% of the strains had a synergistic effect and that 31% of the strains had an additive one. Regardless, other strains with higher MIC values could not be evaluated to ascertain their synergistic effects. For most of the strains (81.5%), apramycin's meropenem rate and MIC value substantially decreased, falling into the lower range (0.5–2 μg/mL). Due to the lower synergistic rate of the colistin/apramycin combination and its adverse nephrotoxic effects, it may be useful to consider the combination of meropenem/apramycin during treatment. Combinations are the only effective treatment option against colistin-resistant strains. Because the meropenem/apramycin combination was highly efficient against these 26 colistin-resistant strains (with a high range of positive synergistic and additive effects), it could prove to be an effective treatment option in the future.

In summary, this study determined that the aztreonam/avibactam combination had a high in vitro synergistic effect against carbapenemase-producing XDR K. pneumoniae strains, such as MBL, and could prove promising for future treatment. The meropenem/apramycin combination was also found to be considerably effective against colistin-resistant strains and noteworthy for subsequent treatment options. Nevertheless, before considering the combination for its treatment potential, future clinical studies should take into consideration the lower synergistic effect that the colistin/apramycin combination produces in colistin-resistant strains and that apramycin on its own results in lower MIC values for these strains. In conclusion, although there are currently limited options for treating carbapenemase-producing K. pneumoniae strains, these drugs may offer new treatment alternatives.

Footnotes

Disclosure Statement

No competing financial interests exist.

Funding Information

This research was funded by Sakarya University Scientific Research Projects Unit (Project Number: 2016-40-02-004).

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