Objective: Salmonella isolates from retail food were examined for antimicrobial susceptibility and further characterized to better understand the development and dissemination of antimicrobial resistance among foodborne Salmonella in China.
Methods: Antimicrobial susceptibility of 359 Salmonella isolates was determined by using agar dilution methods recommended by the Clinical and Laboratory Standards Institute. Antimicrobial resistance integrons and resistance genes were identified using PCR. Mutations in gyrase and Topoisomerase genes related to fluoroquinolones resistance were also determined using PCR and gene sequencing analysis.
Results: Among the 359 Salmonellae isolates, 67% were resistant to Sulfamethoxazole, followed by resistant to trimethoprim/Sulfamethoxazole (58%), tetracycline (56%), kanamycin (37%), nalidixic acid (35%), ampicillin (33%), amoxicillin/clavulanic acid (32%), streptomycin (29%), chloramphenicol and gentamicin (26%), ciprofloxacin (21%), ceftriaxone (16%), cefoxitin (9%) and cefoperazone (8%). Among the 284 resistant isolates, 79% were resistant to at least one antimicrobial, 25.9% to 10 or more than 10 antimicrobials, and 2.5% to 14 antibiotic agents. Integrons were detected in some of sulfamethoxazole-ressitant Salmonella, and the most common integron was 1.4 kb, Antimicrobial resistance genes carried by integrons included aadA1, aadA2, aadA5, tetR, blaPSE1, blaDHA1, blaVEB-1, dhfr I, dhfr V, dhfrVl and dhfr17. The blaTEM gene was also detected in 51.6% of 62 ceftriaxone and/or cefoperazone resistant isolates, and blacMY-2 was detected in 56.5% of the isolates. 13.6% of the Salmonella isolates carried Salmonella Gene Island. Sixty-eight point mutations were detected in gyrA, parC and parE of 35 fluoroquinolone-resistant Salmonella isolates. The common mutations in gyrA gene were Ser83Phe, Ser83Tyr, Asp87Gly and Asp87Asn, whereas ser80Arg was detected in parC. Mutations including Lys441 Ile, Lys428Gln, Asp494Asn, Lys428Gln and Gly442Ser were detected in parE, which was first reported in Salmonella.
Conclusion: Antimicrobial resistance of Salmonella recovered from food in Shaanxi province was common. Several genetic elements including integron, Salmonella Gene Island, beta-lactamase genes and mutations in gyrase and topoisomerase genes played an important role in antimicrobial resistance of Salmonella.