Characterization And Mechanism Of Antimicrobial Resistance Of Foodborne Salmonella

Salmonella is one of the most important foodborne pathogens worldwide. Foods of animal origin are frequently contaminated with the pathogen and can serve as vehicles in transmitting salmonellosis in humans. A survey on Salmonella in retail meats and salads was conducted to determine the prevalence, and phenotypic and genotypic characteristics including serotype, antimicrobial susceptibility, genomic subtype and possession of integronsⅠ, Salmonella Gene IslandⅠ(SGIⅠ) and antimicrobial resistance genes. The food products including chicken, pork, beef, lamb and read to eat salad were collected in Xian, Yangling and Baoji in Shaanxi Province. Additionally, the contributions of several genetic elements such as integronsⅠ, SGIⅠ, plasmids and antimicrobial resistance genes, as well as gene mutation, methyl-directed mismatch repair (MMR) system mutation and defective, to the antimicrobial resistance of Salmonella were investigated. Major findings and conclusions were summarized in the following:(1) Salmonella contamination in chicken, pork, beef, lamb and read to eat salad in supermarket and freemarket in several districts of Shaanxi province was common. The comtamination rates ranged from 9.6% in salad to 69.9% in chicken. The pathogen was also identified in pork (19.4%), beef (30.1%) and lamb (55.3%). In addition, pathogenic Escherichia coli and Staphyloccocus aureus were also detected in these products.(2) The serotypes and genetypes of foodborne Salmonella were diverse. Among 359 Salmonella isolates recovered in the study, 24 serotypes were identified with S.Enteritidis (31.5%) being most common, followed by S. Typhimurium (13.4%), S. Shubra (10.0%) and S. Indiana (9.7%). Several rare serotypes, such as S. Rissen, S. Brancaster, S. Braenderup, S. Litchfield, S. Pakistan and S. Bsilla, were identified as well. ??? of pattens were generated when the chromosome DNA of Salmonella was digested with restriction endonuclease XbalⅠa nd analyzed using pluse-field gel electrophoresis (PFGE). Serotyping and PFGE results were in agreement although PFGE was much discriminatory than serotyping.(3)The Salmonella isolates also exhibited resistance to antimicrobials. Multidrug resistance (MDR) was common. Of the 359 isolates, 67% was resistant to sulfamethoxazole, 56% to tetracycline, 37% to kanamycin, 35% to nalidixic acid, 33% to ampicillin, 32% to amoxicillin / clavulanic acid and 16% to ceftriaxone. One hundred and eighty nine (52.6%) isolates were resistant to at least four antimicrobials, 93 (25.9%) to≥10 or more, 33 (9.2%) to≥11 and 25 (7.0%) to≥12, 17 (4.7%) to≥13, and 9 (2.5%) to≥14 antimicrobials. The common serotypes of MDR Salmonella included SalmonellaⅡ, S. Rideau, S. Saintpaul, S. Shubra, S. Indiana, S. Typhimurium and S. Infantis.(4) Antimicrobial resistance encoded by mobile genetic elements, such as integronsⅠand SGIⅠ, were important in mediating MDR of Salmonella.Of 359 Salmonella isolates recovered from Shaanxi, China and 390 from U.S.A, integronsⅠw ere found in 16.0% and 17.3% isolates, and SGIⅠin 13.6% and 7.4%, respectively. All integrons were detected in MDR Salmonella isolates. Genes carried in integronsⅠgene cassettes included aadA1, aadA2, aadA5, tetR, blaPSE-1, blaDHA-1, blaVEB-1, dhfr1, dhfrⅤ, dhfrⅦand dhfr17. Isolates with SGIⅠexhibited relatively extended antimicrobial resistance spectrum, and showed resistant to not only penicillin and cephalosporin, such as ampicillin, ceftriaxone and cefoperazone, but also aminoglycosides and fluoroquinolones, such as gentamicin, kanamycin, amikacin, streptomycin and ciprofloxacin, as well as resistance to chloramphenicol.BlaTEM gene was detected in 32 (8.9%) and blaCMY-2 in 35 (9.7%) Salmonella isolates that were resistant to ampicillin, and to ceftriaxone and/or cefoperazone, and/or cefoxitin, indicating that ESBLs encoded by blaTEM and/or blaCMY-2 were responsible for the resistance.(5) Mutations in Salmonella gyrase and topoisomerase genes cause decreased susceptibility to fluouoquinolones and multidrug resistance. Of 30 MDR Salmonella isolates, 4 (13.3%) exhibited resistance to all seven fluoroquinolones tested, 16 (53.3%) to 6, 8 (26.6%) to 3 fluoroquinolones. A total of 68 mutations in gyrA, parC and parE were found in the 30 Samonella isolates, whereas no gyrB mutation was detected. The most common mutations were Ser83→Phe, Asp87→Gly and Asp87→Asn in gyrA, and Ser80→Arg in parC gene. Four of the isolates had novel mutations in parE, including Lys427→Gln, Lys441→Ile, Gly442→Ser and Asp494→Asn. Isolates with single gyrA mutations were less resistant to fluoroquinolones than those together with an additional parC and/or parE mutation in Salmonella.(6) MMR system gene mutation in Salmonella is an important mechanism of antimicrobial resistance; however, the incidence of hypermutator due to defective in MMR in Salmonella seems rare. In this study, five presumptive hypermutators, S. Typhimurium ST20751, S. Heidelberg 22396, S. Enteritidis 17929, 17929N and 17929R, were identified after screening 390 Salmonella isolated from U.S.A. Wild-type mutH, mutL, mutS and uvrD complementation and DNA sequencing study demonstrated no defective or sense mutation occurred in MMR genes, and the probability of hypermutation in the chromosome DNA that resulted in MDR in Salmonella were rare.(7) Plasmid plays an important role development and dissemination of antimicrobial resistance. Endonuclease digested plasmid profiles and clustering analysis indicated that antimicrobial resistance encoded by plasmid was able to be transmitted to different bacteria among through horizontal gene transfer. The conjugation ratios of resistance gene transfer to the recipients (EC1003 and 17929N) were from 2.4×10-4 to 5.6×10-4. After conjugation, most transconjugants acquired the corresponding antimicrobial resistant phenotypes. The antimicrobial genes transferred during the conjugation were verified by PCR, and conferred the transconjugant new antimicrobial resistance phenotype.