Antibiotic Susceptibility and Molecular Screening of Class I Integron in Salmonella Isolates Recovered from Retail Raw Chicken Carcasses in China

Abstract

Salmonella is one of the leading causes for foodborne diseases. Foods, particularly those of animal origin, act as an important role for Salmonella transmission. In this study, the antibiotic susceptibility of 743 Salmonella isolates recovered from retail raw chicken carcasses in eight provinces was tested, and the isolates were also screened for the presence of class I integron and drug-resistant gene cassettes. One hundred thirteen (15.21%) isolates were harboring class I integron. A higher percentage of integron-positive Salmonella isolates were found in retail chicken in Sichuan Province (29.33%), followed by Beijing (22.14%), Shaanxi (19.15%), Guangxi (14.13%), Henan (12.50%), Shanghai (7.25%), Fujian (8.22%), and Guangdong (6.25%) Provinces. The respective prevalence of class I integron in Salmonella isolates recovered from retail chickens in large, free, and small markets was 16.31%, 14.04%, and 15.27%. Moreover, 20.13%, 14.02%, and 13.74% of Salmonella isolates recovered from retail chickens stored in frozen, chilled, and ambient conditions, respectively, were positive for class I integron. Subsequent sequencing of class I integron revealed the presence of 10 gene cassettes harboring resistance genes (dfrA17-aadA5, dfrA17-aadA5, dfrA1-aadA1, dfrA12-aadA2, dfrA17-aadA5-aadA4, dfrA1-aadA1-aadA2, dfrA1, dfrA5, aadA2, aacA4-catB8-aadA1-dfrA1-(aac6-II)-(bla_CARB-8), bla_PSE-1-bla_P1). The most prevalent gene cassette was dfrA17-aadA5 (59.62%). Class I integron-positive isolates were significantly more resistant to multiple antibiotics, and they commonly exhibited corresponding antibiotic resistance profiles to the antibiotic resistance gene cassettes harbored in their class I integron. The results indicated that class I integron with different antibiotic resistance gene cassettes that were prevalent in Salmonella isolates differed from provinces, marketplaces, and chicken storage conditions.

Introduction

Salmonella is a notorious pathogen associated with foodborne outbreaks worldwide.¹ Salmonella infections were reported as the second leading cause of bacterial foodborne illness in the United States² and were responsible for ∼1.2 million patients annually, resulting in estimated medical costs of $365 million.³ In the European Union, salmonellosis was the second most commonly reported gastrointestinal infection, with a confirmed case rate of 20.4 cases per 100,000 individuals in 2011.⁴ In China, 22.2% of foodborne diseases were caused by Salmonella spp.,⁵ and salmonellosis was ranked as the fourth most prevalent foodborne disease caused by microbial agents.⁶

Among various foods, poultry and poultry products (meat and eggs) had been recognized as the most significant vehicle for human salmonellosis,⁷ and several foodborne Salmonella outbreaks were caused by contaminated poultry products.^8,9 Antibiotics would remain the most effective treatment for invasive, life-threatening salmonellosis infection for a long time in the future.¹⁰ However, ubiquitous usage of antibiotics not only facilitated the emergence of antibiotic resistance but also created significant challenges to animal husbandry.^10,11 Several previous foodborne outbreaks indicated that the multidrug-resistant (MDR) Salmonella was of increasing concern.^12–15

Integrons are mobile DNA elements comprising a site-specific recombination system that is capable of integrating and expressing antibiotic resistance genes in cassette-like structures and transferring resistant genes from one bacterium to another, which in turn lead to the emergence of antibiotic resistance.^15,16 Among various integrons, class I integron was the most predominant one and played a major role in multidrug resistance development of Salmonella.^17–20

Several previous studies indicated that class I integrons could be detected in MDR Salmonella from animals, eggs, poultry, human, pork, retail beef, and chicken carcasses in Japan, the United States, Iran, Ethiopia, Taiwan, Brazil, and China.^{8,12,16,17,19–22} However, information regarding the distribution of antimicrobial resistance-associated integron in Salmonella among retail foods is still not well documented. In this study, the prevalence of class I integron and antibiotic resistance-associated gene cassettes in Salmonella isolates were determined for better understanding class I integron distribution in Salmonella isolates recovered from retail chicken carcasses in China.

Materials and Methods

Salmonella isolates

Seven hundred forty-three Salmonella isolates (73 isolates were from Fujian, 75 from Sichuan, 88 from Henan, 92 from Guangxi, 94 from Shaanxi, 112 from Guangdong Province, 69 from Shanghai, and 140 from Beijing) were included in this study. These isolates were recovered from retail raw chicken carcasses collected from different marketplaces in six provinces and two national cities in China during 2010 and 2011. Detailed information on sample collection, Salmonella isolation, and identification were as previously described.^23,24

Antimicrobial susceptibility test

All Salmonella isolates were tested for their susceptibility to antimicrobials. The minimum inhibitory concentrations (MICs) of the antimicrobial agents were determined by the agar dilution method using Mueller–Hinton agar (Beijing Land Bridge Technology Co Ltd., Beijing, China) according to the guidelines recommended by the Clinical and Laboratory Standards Institute (CLSI).²⁵ The antibiotics include ampicillin (AMP), amoxicillin–clavulanic acid (AMC), ceftiofur (TIO), ceftriaxone (CRO), cefoxitin (FOX), gentamicin (GEN), kanamycin (KAN), amikacin (AMK), streptomycin (STR), tetracyclines (TCY), sulfisoxazole (FIS), trimethoprim–sulfamethoxazole (SXT), chloramphenicol (CHL), nalidixic acid (NAL), and ciprofloxacin (CIP). Escherichia coli ATCC25922 and ATCC35218, Enterococcus faecalis ATCC29212 were used as quality control organisms in MIC determination. The breakpoints for the antibiotics were as the interpretive standards provided by CLSI,²⁵ except for streptomycin, for which we used provisional breakpoint used by the National Antimicrobial Resistance Monitoring System (NARMS).²⁶

Molecular detection of class I integron

All Salmonella isolates were screened for the presence of class I integron by PCR using the primers of intI-F-(5′CS)-5′-GGCATCCAAGCAGCAAG-3′ and intI-R-(3′CS)-5′-AAGCAGACTTGACCTGA-3′.^27,28 The PCRs were carried out in a Mycircle PCR system (Bio-Rad, Hercules, CA) in a 25 μl PCR mixture that contained 0.5 μM of each primer, 1 × PCR buffer, 250 μM of dNTP, 0.5 U of Taq DNA polymerase (TaKaRa, Dalian, China), 1.5 mM MgCl₂, and 5 μl of sample template DNA. The PCR conditions were at 94°C for 10 min, followed by 35 cycles of 94°C for 1 min, 56°C for 1 min and 72°C for 1 min, and a final extension of 72°C for 10 min.

DNA sequencing and resistance gene cassette analysis

PCR products were purified using the TaKaRa Agarose Gel DNA Purification Kit (Version 2.0; TaKaRa), then stored in an ice box, and sent for sequencing at Shanghai Sunny Biotechnology Co., Ltd. (Shanghai, China). DNA sequence data were analyzed and aligned using BLAST program (http://ncbi.nlm.nih.gov/BLAST/).

Statistical analysis

The prevalence data of class I integron in Salmonella isolates were subjected to DPS software (version 9.5, Institute of Insect Science, Zhejiang University, Hangzhou, People's Republic of China) to determine the significant variation. Significant (p < 0.05) difference of the prevalence of class I integron in Salmonella isolates in different provinces and cities, different retail chicken storage conditions, and different marketplaces was determined using the Pearson's chi-squared test at the 5% level (α = 0.05).

Results

Among 743 Salmonella isolates, a higher percentage of the isolates (77.12%) were resistant to tetracycline, followed by sulfisoxazole (66.62%), nalidixic acid (64.74%), ampicillin (61.37%), trimethoprim–sulfamethoxazole (56.53%), and amoxicillin–clavulanic acid (48.45%) (Table 1). One hundred thirteen (15.2%) of the total isolates were harboring class I integron of 2 kb. Salmonella isolates harboring class I integron presented a significantly (p < 0.05) higher resistance to tetracycline, ampicillin, trimethoprim–sulfamethoxazole, amoxicillin–clavulanic acid, chloramphenicol, kanamycin, gentamicin, ceftiofur, cefoxitin, and amikacin compared with the average resistance rates (Table 1).

Table 1.

Antimicrobial Resistance of the Salmonella Isolates

Antimicrobial agent	No. (%) of antimicrobial-resistant isolates (n = 743)	No. (%) of antimicrobial-resistant isolates carrying class I integron (n = 113)
Aminoglycosides
Amikacin	138 (18.57) A	44 (38.94) B
Gentamicin	269 (36.20) A	60 (53.10) B
Kanamycin	309 (41.59) A	63 (55.75) B
Streptomycin	223 (30.01) A	29 (25.66) A
Cephalosporins
Ceftiofur	193 (25.98) A	53 (46.90) B
Cefoxitin	141 (18.98) A	43 (38.05) B
Ceftriaxone	126 (16.96) A	22 (19.47) A
Penicillin β-lactamase inhibitor combinations
Ampicillin	456 (61.37) A	85 (75.22) B
Amoxicillin–clavulanic acid	360 (48.45) A	78 (69.03) B
Quinolone
Nalidixic acid	481 (64.74) A	56 (49.56) B
Fluoroquinolones
Ciprofloxacin	170 (22.88) A	16 (14.16) B
Gatifloxacin	129 (17.36) A	15 (13.27) A
Levofloxacin	135 (18.17) A	16 (14.16) A
Folate pathway inhibitors
Sulfisoxazole	495 (66.62) A	78 (69.03) A
Trimethoprim–sulfamethoxazole	420 (56.53) A	83 (73.45) B
Chloramphenicol
Chloramphenicol	327 (44.01) A	76 (67.26) B
Tetracycline
Tetracycline	573 (77.12) A	100 (88.50) B

Ratios followed by different letters were significantly different (p < 0.05).

Table 2 clarified that in all 113 class I integron-positive isolates, 5 (4.42%), 23 (20.35%), 37 (32.74%), 37 (32.74%), and 8 (7.08%) isolates showed resistance to 0, 2–5, 6–9, 10–13, and 14–17 tested antibiotics, respectively. Class I integron-positive isolates presented significantly (p < 0.05) more resistance to multiple antibiotics compared with the average rates of Salmonella.

Table 2.

Multidrug Resistance of Salmonella Isolates Against 17 Antimicrobials

Number of antimicrobials	No. (%) of isolates resistant to indicated number of antimicrobials (n = 743)	No. (%) of class I integron-positive isolates resistant to indicated number of antimicrobials (n = 113)
0	38 (5.11) A	5 (4.42) A
1	43 (5.79) A	3 (2.65) A
2–5	284 (38.22) A	23 (20.35) B
6–9	177 (23.82) A	37 (32.74) B
10–13	111 (14.94) A	37 (32.74) B
14–17	90 (12.11) A	8 (7.08) B

Ratios followed by different letters were significantly different (p < 0.05).

The results clarified a higher prevalence rate (29.33%) of class I integron-positive Salmonella isolates recovered from retail chicken carcasses in Sichuan Province and this rate was significantly higher (p < 0.05) than those in Guangxi (14.13%), Henan (12.50%), Fujian (8.22%), Shanghai (7.25%), and Guangdong (6.25%) Provinces. Moreover, there was no significant difference (p > 0.05) in the detection rates of class I integron-positive isolates in Sichuan (29.33%), Beijing (22.14%), and Shaanxi (19.15%) Provinces. Collectively, our results showed a significantly (p < 0.05) higher prevalence rate (20.65%) of class I integron-positive Salmonella isolates in the Northern Provinces of China (Henan, Shaanxi, Sichuan, and Beijing) compared to (8.96%) the Southern ones (Fujian, Guangdong, Guangxi, and Shanghai).

No significant difference was found among the prevalence rates of Class I integron in Salmonella isolates recovered from retail chickens stored at frozen (20.13%), chilled (14.02%), and ambient (13.74%) conditions. In addition, class I integron was more prevalent in Salmonella strains isolated from chickens stored at frozen conditions in Beijing (10.39%) and less prevalent in Henan Province (0.65%). On the contrary, class I integron was found in Salmonella recovered from chicken carcasses stored at chilled conditions in the Sichuan Province (2.38%) and 3.79% in chickens stored at ambient temperature in the Beijing Province.

Regarding the distribution of class I integron in Salmonella spp. isolated from chickens at different marketplaces, Table 3 clarified that 16.31%, 15.27%, and 14.04% of the total isolates recovered from chickens at large, free, and small markets, respectively, were harboring class I integron. In addition, in Salmonella strains isolated from chickens of large markets, class I integron was more prevalent in Shaanxi Province (3.86%) than in other Provinces. Meanwhile, for small markets and free markets, the highest prevalence rates of class I integron were both detected in Beijing, with a ratio of 5.96% and 4.03%, respectively. Moreover, there was no significant difference (p > 0.05) in the detection rates of class I integron-positive isolates between capital cities and noncapital cities (Table 3).

Table 3.

Distribution of Class I Integron in Salmonella Isolates Recovered from Retail Chickens Collected in Different Districts, Marketplaces, and Storage Conditions

	No. isolates	No. class I integron-positive isolates	% class I integron-positive isolates
Province/national city
Sichuan	75	22	29.33 A
Beijing	140	31	22.14 AB
Shaanxi	94	18	19.15 AB
Guangxi	92	13	14.13 BC
Henan	88	11	12.50 BC
Fujian	73	6	8.22 C
Shanghai	69	5	7.25 C
Guangdong	112	7	6.25 C
Chicken storage condition
Frozen	154	31	20.13 A
Chilled	378	53	14.02 A
Ambient	211	29	13.74 A
Marketplace
Large market	233	38	16.31 A
Small market	235	33	14.04 A
Free market	275	42	15.27 A
Capital city/noncapital city
Capital city	161	24	14.91 A
Noncapital city	582	89	15.29 A
Northern/Southern province (city) of China^a
Southern province (city)	346	31	8.96 B
Northern province (city)	397	82	20.65 A

Ratios followed by different letters were significantly different (p < 0.05).

Northern provinces (city), including Shaanxi, Sichuan, Henan Provinces, and Beijing city; Southern provinces (city), including Guangdong, Guangxi, Fujian Provinces, and Shanghai city.

Among the 113 class I integron-positive Salmonella isolates, nine (7.96%) were carrying no antibiotic-resistant gene cassettes (Table 4). Approximately 8.85% of class I integron carried one antibiotic resistance gene, and 83.19% of class I integron simultaneously carried two or more resistance genes. In addition, 10 different gene cassettes containing 14 resistance genes were identified in 104 (92.04%) of Salmonella isolates. The cassettes included dfrA17-aadA5, dfrA1-aadA1, bla_PSE-1-bla_P1, dfrA1, aadA2, dfrA5, dfrA12-aadA2, aacA4-catB8-aadA1-dfrA1-(aac6-II)-(bla_CARB-8), dfrA1-aadA1-aadA2, and dfrA17-aadA5-aadA4, respectively (Table 4). The most prevalent resistance gene cassette was dfrA17-aadA5 (54.87%), followed by dfrA1-aadA1 (21.24%) and dfrA1 (6.19%).

Table 4.

Presence of Antibiotic Resistance Gene Cassettes in Class I Integron-Positive Salmonella in Retail Chicken Carcasses (n = 113)

Antibiotic resistance gene cassettes	No. class I integron-positive isolates	% class I integron- positive isolates
dfrA17-aadA5	62	54.87
dfrA1-aadA1	24	21.24
dfrA1	7	6.19
dfrA12-aadA2	3	2.65
aadA2	2	1.77
bla_PSE-1-bla_P1	2	1.77
dfrA5	1	0.88
aacA4-catB8-aadA1-dfrA1-(aac6-II)-(bla_CARB-8)	1	0.88
dfrA1-aadA1-aadA2	1	0.88
dfrA17-aadA5-aadA4	1	0.88
	9	7.96

The results clarified that among class I integron-positive Salmonella isolates with dfr genes (dfrA1, dfrA5, dfrA12, and dfrA17), the predominant antimicrobial resistance profile was FIS-SXT (65.22%). Meanwhile, 31.40% of isolates harboring aad genes (aadA1, aadA2, aadA5, and aadA4) presented the profile of GEN-KAN-AMK. Among 11 class I integron-positive Salmonella isolates presenting antibiotic resistance profile of GEN-KAN-AMK-STR, at least one of the aad genes could be detected. Meanwhile, two class I integron-positive Salmonella isolates carrying bla_PSE-1-bla_P1 genes were resistant to ceftiofur and ceftriaxone.

Discussion

Salmonella is a major cause of foodborne illness worldwide. The increasing frequency of antibiotic resistance among Salmonella has become an emerging challenge to public health.²⁹ In the present study, 89.10% of Salmonella isolates were resistant to two or more antimicrobial agents. These results were similar to those of other investigators.^30–32 Our study indicated that Salmonella isolates were most frequently resistant to tetracycline (77.12%), sulfisoxazole (66.62%), and nalidixic acid (64.74%), and this was consistent with the findings of other studies.^33,34 This study presented that 92.92% of the class I integron-positive isolates were resistant to at least two antibiotics, and this exhibited a good evidence for the relationship between the presence of class I integron and the emerging of MDR in gram-negative bacteria.^12,29

Integron, a mobile DNA element, is known to contain one or more linked antimicrobial resistance genes and can transfer these genes among bacteria.^35,36 In this study, PCR analysis showed that 113 (15.21%) of 743 Salmonella isolates were carrying class I integron. In one of our previous studies, it was found that 16.0% of the Salmonella strains isolated from retail foods carried class I integron in Shaanxi province during the period 2007–2008.³⁷ Other previous surveys in China identified class I integron in Salmonella isolates recovered from retail foods, human, and animal sources with percentages that ranged from 11.0% to 66.0%.^21,22,38,39 Compared with these surveys, we were the first to investigate class I integron in Salmonella isolates in retail raw chickens by such large sample size and basis.

In the present study, there was a variation in the prevalence of class I integron in Salmonella isolates recovered from retail chickens of different provinces, national cities, storage conditions, and marketplaces. This variation could be attributed to the difference in the growth and slaughter environment of chickens.³⁸ In addition, medical qualities had certain varieties, which led to the difference of class I integron and resistance genes.⁴⁰ Chicken carcasses were often stored in the markets as fresh at ambient, or as packaged chilled products, or in frozen conditions. The difference in temperature, in which chickens were stored, might have influenced the number and types of Salmonella, and consequently affected the popularity of class I integron. In our study, class I integron was most frequently prevalent in Salmonella recovered from chicken carcasses that were stored in frozen conditions (20.13%) compared to 13.74% in chickens stored at ambient temperature. The previous studies had shown that Salmonella contamination in the raw chickens was related to temperature,⁴¹ and our results indicated that the survived Salmonella isolates in frozen chicken carcass were more resistant and likely to be class I integron positive.

The former studies have indicated the existence of certain relationship between integrons and multidrug resistance of Salmonella strains, and provided certain significant guidelines for drug resistance prevention and control.^20,29 However, it was interesting that no resistance gene was detected in the variable region of class I integron in the two MDR isolates in this study. As it was known, antibiotic resistance associated with many factors, including antimicrobial agents binding target mutations, increased permeability of membrane of the pathogens, and antibiotic degradation through enzymes encoded by some resistant genes that were commonly carried by some transferable DNA elements, including conjugative plasmids, integrons, transposons, and phages.^{9,18,21,35,42} Since any transferable element other than class I integron could play an important role for antimicrobial resistance gene carriage and dissemination as well, the resistance of the two isolates might be caused by other mechanisms instead of class I integron.^16,27,30

Our results indicated that the isolates harboring class I integron were more resistant to the antibiotics. The dfr, aad, and bla genes have always contributed to the resistance to sulfonamide, aminoglycoside, and cephalosporin antibiotics, respectively. In this study, 91.3% of the 92 isolates carrying dfr genes (dfrA1, dfrA5, dfrA12, and dfrA17) showed resistance to sulfonamides. The resistance to aminoglycoside antibiotics was identified in 51 of the 86 isolates carrying aad genes (aadA1, aadA2, aadA5, and aadA4). In addition, two of the class I integron-positive isolates carrying bla_PSE-1-bla_P1 genes were resistant to ceftiofur and ceftriaxone. Various antibiotic resistance gene cassettes, including dfrA17-aadA5, dfrA1-aadA1, bla_PSE-1-bla_P1, dfrA1, aadA2, dfrA5, dfrA12-aadA2, aacA4-catB8-aadA1-dfrA1-(aac6-II)-(blaCARB-8), dfrA1-aadA1-aadA2, and dfrA17-aadA5-aadA4, were found in integron, and Salmonella isolates with these gene cassettes in their class I integron generally exhibited corresponding antibiotic resistance profiles. The results inferred that the gene cassettes in the class I integron might be the major cause of antibiotic resistance in those Salmonella. In class I integron, antibiotic resistance gene cassettes were normally found integrated at a specific location, and enzymes encoded by the integron could make the gene cassettes transfer from one bacteria to another, or from one strain to other strains.^42–44 The presence of class I integron in Salmonella isolates is of great potential hazard to the health and food safety of humans and animals.

In summary, class I integron was ubiquitous in Salmonella isolates recovered from retail raw chicken carcasses and varied in different marketplaces and chicken storage conditions at different districts of China. Salmonella isolates harboring class I integron showed more resistance to the antibiotics than those without it. Two or more antibiotic resistance gene cassettes were prevalent in class I integron.

Footnotes

Acknowledgment

The research work was supported by the National Natural Science Foundation of China (No. 31171682).

Disclosure Statement

No competing financial interests exist.

References

World Health Organization. 2005. Drug-resistant Salmonella. Available at http://who.int/mediacentre/factsheets/fs139/en/ (assessed on June 23, 2015). (Online.)

Scallan

, Hoekstra

R.M.

, Angulo

F.J.

, Tauxe

R.V.

, Widdowson

M.A.

, Roy

S.L.

, Jones

J.L.

, and Griffin

P.M.

2011. Foodborne illness acquired in the United States—major pathogens. Emerg. Infect. Dis., 17:7–15.

Centers for Disease Control and Prevention (CDC). 2014. Antibiotic Resistance Threats in the United States, 2013. 73. Available online at http://cdc.gov/drugresistance/threat-report-2013/pdf/ar-threats-2013–2508.pdf (assessed on June 18, 2015). (Online.)

European Centre for Disease Prevention and Control. 2013. Annual Epidemiological Report Reporting on 2011 Surveillance Data and 2012 Epidemic Intelligence Data. 103–108. Available online at: http://ecdc.europa.eu/en/publications/_layouts/forms/Publication_DispForm.aspx?List=4f55ad51-4aed-4d32-b960-af70113dbb90&ID=989 (assessed on June 23, 2015). (Online.)

Wang

S.J.

, Duan

H.L.

, Zhang

, and Li

2007. Analysis of bacterial food-borne diseases outbreaks in China between 1994 and 2005. FEMS Immunol. Med. Microbiol., 51:8–13.

Zhu

, Cui

, Lin

, Xu

, Zhao

, Xia

, Deng

, Xie

, Zhang

, Wang

, Feng

, Yang

, and Ran

2012. Analysis of the aetiology of diarrhea in outpatients in 2007, Henan province, China. Epidemiol. Infect., 7:540–548.

Painter

J.A.

, Hoekstra

R.M.

, Ayers

, Tauxe

R.V.

, Braden

C.R.

, Angulo

F.J.

, and Griffin

P.M.

2013. Attribution of food-borne illnesses, hospitalizations, and deaths to food commodities by using outbreak data, United States, 1998–2008. Emerg. Infect. Dis., 19:407.

Kusunoki

, Jin

, Iwaya

, Ishikami

, Kurimoto

, Saito

, and Yamada

2000. Salmonella contamination in domestic raw chickens in Tama, Tokyo, and serovar or drug-resistance of isolates (1992–1999). Jpn. J. Food. Microbiol., 17:207–212.

Zou

, Frye

J.G.

, Chang

C.W.

, Liu

, Cerniglia

C.E.

, and Nayak

2009. Microarray analysis of antimicrobial resistance genes in Salmonella enterica from preharvest poultry environment. J. Appl. Microbiol., 107:906–914.

10.

Ribeiro

V.B.

, Lincopan

, Landgraf

, Franco

B.D.

, and Destro

M.T.

2011. Characterization of class I integrons and antibiotic resistance genes in multidrug-resistant Salmonella enterica isolates from foodstuff and related sources. Braz. J. Microbiol., 42:685–692.

11.

Yang

, Qiao

, Zhang

, Cui

, Xia

, Cui

, Wang

, Meng

, Ge

, Shi

, Wang

, and Meng

2013. Serotyping, antimicrobial susceptibility, pulse field gel electrophoresis analysis of Salmonella isolates from retail foods in Henan Province, China. Food Control, 32:228–235.

12.

Asgharpour

, Rajabnia

, Shahandashti

E.F.

, Marashi

M.A.

, Khalilian

, and Moulana

2014. Investigation of class I integron in Salmonella infantis and its association with drug resistance. Jundishapur. J. Microbiol., 7:e10019.

13.

Dechet

A.M.

, Scallan

, Gensheimer

, Hoekstra

, Gunderman-King

, Lockett

, Wrigley

, Chege

, and Sobel

2006. Outbreak of multidrug-resistant Salmonella enterica serotype Typhimurium definitive type 104 infection linked to commercial ground beef, northeastern United States, 2003–2004. Clin. Infect. Dis., 42:747–752.

14.

McLaughlin

J.B.

, Castrodale

L.J.

, Gardner

M.J.

, Ahmed

, and Gessner

B.D.

2006. Outbreak of multidrug-resistant Salmonella Typhimurium associated with ground beef served at a school potluck. J. Food Prot., 69:666–670.

15.

Wannaprasat

, Padungtod

, and Chuanchuen

2011. Class I integrons and virulence genes in Salmonella enterica isolates from pork and humans. Int. J. Antimicrob. Agents, 37:457–461.

16.

Peirano

, Agersø

, Aarestrup

F.M.

, dos Reis

E.M.F.

, and dos Prazeres Rodrigues

2006. Occurrence of integrons and antimicrobial resistance genes among Salmonella enterica from Brazil. J. Antimicrob. Chemother., 58:305–309.

17.

Ahmed

A.M.

, Ishida

, and Shimamoto

2009. Molecular characterization of antimicrobial resistance in Salmonella isolated from animals in Japan. J. Appl. Microbiol., 106:402–409.

18.

Chu

, and Chiu

C.H.

2006. Evolution of the virulence plasmids of non-typhoid Salmonella and its association with antimicrobial resistance. Microbes Infect., 8:1931–1936.

19.

Hsu

S.C.

, Chiu

T.H.

, Pang

J.C.

, Hsuan-Yuan

C.H.

, Chang

G.N.

, and Tsen

H.Y.

2006. Characterisation of antimicrobial resistance patterns and class I integrons among Escherichia coli and Salmonella enterica serovar Choleraesuis strains isolated from humans and swine in Taiwan. Int. J. Antimicrob. Agents, 27:383–391.

20.

, Zhao

, Zhang

, Hou

, and Wu

2014. Survey of multidrug resistance and class I integron prevalence in chicken-derived Salmonella. Chin. Agric. Sci. Technol., 15:877–881.

21.

Lian

, Shi

, Mao

, Sun

, and Wang

2013. The study of integron mediated antibiotic resistance transmit of Salmonella. Chin. J. Food Hyg., 1:008.

22.

Molla

, Miko

, Pries

, Hildebrandt

, Kleer

, Schroeter

, and Helmuth

2007. Class I integrons and resistance gene cassettes among multidrug resistant Salmonella serovars isolated from slaughter animals and foods of animal origin in Ethiopia. Acta Trop., 103:142–149.

23.

Cui

, Zheng

, and Meng

2006. An improved method for rapid isolation of Salmonella against Proteus in chicken carcasses. J. Food Saf., 26:49–61.

24.

Yang

, Xi

, Wang

, Cui

, Yue

, Hao

, Wang

, Cui

, Alali

W.Q.

, Meng

, Walls

, Doyle

M.P.

2011. Prevalence of Salmonella on raw poultry at retail markets in China. J. Food Prot., 74:1724–1728.

25.

Clinical and Laboratory Standards Institute. 2010. Performance Standards for Antimicrobial Susceptibility Testing 2010, Twentieth International Ed. Document M100-S20. Pennsylvania, USA.

26.

U.S. Food and Drug Administration. 2013. NARMS retail meat annual report, 2011. U.S. Food and Drug Administration, Rockville, MD. Available at http://fda.gov/downloads/AnimalVeterinary/SafetyHealth/AntimicrobialResistance/NationalAntimicrobialResistance MonitoringSystem/UCM334834.pdf (assessed on June 20, 2015). (Online.)

27.

White

D.G.

, Zhao

, Sudler

, Ayers

, Friedman

, Chen

, McDermott

P.F.

, McDermott

, Wagner

D.D.

, and Meng

2001. The isolation of antibiotic-resistant Salmonella from retail ground meats. N. Engl. J. Med., 345:1147–1154.

28.

Yang

, Qu

, Zhang

, Shen

, Cui

, Shi

, Xi

, Sheng

, Zhi

, and Meng

2010. Prevalence and characterization of Salmonella serovars in retail meats of marketplace in Shaanxi, China. Int. J. Food Microbiol., 141:63–72.

29.

Firoozeh

, Zahraei-Salehi

, Shahcheraghi

, Karimi

, and Aslani

M.M.

2012. Characterization of class I integrons among Salmonella enterica serovar Enteritidis isolated from humans and poultry. FEMS Immunol. Med. Microbiol., 64:237–243.

30.

Chen

, Zhao

, White

D.G.

, Schroeder

C.M.

, Lu

, Yang

, McDermott

P.F.

, Ayers

, and Meng

2004. Characterization of multiple-antimicrobial-resistant Salmonella serovars isolated from retail meats. Appl. Environ. Microbiol., 70:1–7.

31.

Dogru

A.K.

, Ayaz

N.D.

, and Gencay

Y.E.

2010. Serotype identification and antimicrobial resistance profiles of Salmonella spp. isolated from chicken carcasses. Trop. Anim. Health Prod., 42:893–897.

32.

Lay

K.S.

, Vuthy

, Song

, Phol

, and Sarthou

J.L.

2011. Prevalence, numbers and antimicrobial susceptibilities of Salmonella serovars and Campylobacter spp. in retail poultry in Phnom Penh, Cambodia. J. Vet. Med. Sci., 73:325–329.

33.

Morshed

, and Peighambari

S.M.

2010. Drug resistance, plasmid profile and random amplified polymorphic DNA analysis of Iranian isolates of Salmonella enteritidis. New Microbiol., 33:47–56.

34.

Rahmani

, Peighambari

S.M.

, Svendsen

C.A.

, Cavaco

L.M.

, Agersø

, and Hendriksen

R.S.

2013. Molecular clonality and antimicrobial resistance in Salmonella enterica serovars Enteritidis and Infantis from broilers in three Northern regions of Iran. BMC Vet. Res., 9:66.

35.

Hall

R.M.

, and Stokes

H.W.

1993. Integrons: novel DNA elements which capture genes by site-specific recombination. Genetica., 90:115–132.

36.

Hall

R.M.

1997. Mobile gene cassettes and integrons: moving antibiotic resistance genes in gram-negative bacteria. Ciba Found Symp., 207:192–202; discussion 202–205.

37.

Shen

, Yang

, Xi

, and Meng

2011. Class I integron and antibiotic resistance of Salmonella and Proteus mirabilis in retail meat from Shaanxi Province. Food Sci., 32:130–134.

38.

Chen

, Wu

C.M.

, and Shen

J.Z.

2008. Detection of antimicrobial resistance and class I integrons among Salmonella isolates from animals. Chin. J. Vet. Med., 44:6–9.

39.

Zhou

, Li

, Su

, Li

, and Xu

2014. Detection of class I integron from food-borne Salmonella and analysis on the drug resistance gene cassettes. Sci. Technol. Food Ind., 35:279–282.

40.

Skurnik

, Ruimy

, Andremont

, Amorin

, Rouquet

, Picard

, and Denamur

2006. Effect of human vicinity on antimicrobial resistance and integrons in animal faecal Escherichia coli. J. Antimicrob. Chemother., 57:1215–1219.

41.

Dong

, Chen

, and Luo

2005. Situation of Salmonellae contamination in eggs and poultry on Beijing market in year 2002. Chin. J. Food Hyg., 16:514–516.

42.

Hall

R.M.

, and Collis

C.M.

1995. Mobile gene cassettes and integrons: capture and spread of genes by site-specific recombination. Mol. Microbiol., 15:593–600.

43.

Hall

R.M.

, and Collis

C.M.

1998. Antibiotic resistance in gram-negative bacteria: the role of gene cassettes and integrons. Drug Resist. Updat., 1:109–119.

44.

Stokes

H.T.

, and Hall

R.M.

1989. A novel family of potentially mobile DNA elements encoding site-specific gene-integration functions: integrons. Mol. Microbiol., 3:1669–1683.