Prevalence and Characterization of Antimicrobial Resistance in Salmonella enterica Isolates from Retail Foods in Shanghai, China

This study characterized the prevalence and antimicrobial resistance characteristics of foodborne Salmonella isolates from March 2016 to February 2017 in Shanghai, China. A total of 147 (14.2%) nonduplicate foodborne Salmonella isolates were obtained from 1035 food samples. The Salmonella isolates were most frequently identified in fresh meat samples (28.0%), followed by ready-to-eat foods (9.0%), frozen convenience foods (7.1%), and fresh produce (4.5%). The top 3 serovars were Salmonella Enteritidis (46.3%; 68/147), Salmonella Typhimurium (32.7%; 48/147), and Salmonella Derby (6.8%; 10/147). The majority of isolates were resistant to sulfisoxazole (93.9%; 138/147) and trimethoprim/sulfamethoxazole (61.2%; 90/147). Interestingly, frozen convenience food isolates exhibited an extremely high multidrug resistance rate (86.7%; resistant to ≥3 classes of antimicrobials). Among 81 quinolone-resistant isolates, aac(6′)-Ib-cr (100%), oqxAB (84.0%), qnrS1 (23.5%), D87Y (49.4%), and D87N (33.3%) mutations in GyrA, and T57S in ParC (12.3%) were observed. The β-lactamase genes blaTEM-1 (100%) were present in 63 ampicillin-resistant isolates. Polymerase chain reaction-based plasmid replicon typing revealed that 147 isolates represented 6 plasmid incompatibility groups (IncFIIs, IncHI2, IncI1, IncP, IncFIC, and IncA/C), among which, IncFIIs (59.2%) and IncHI2 (26.5%) were predominant. The genetic relationship of isolates was elucidated using multilocus sequence typing (MLST) and pulsed-field gel electrophoresis (PFGE). MLST results indicated that ST34 and ST11 were predominate types in Salmonella Typhimurium (56.3%; 27/48) and Salmonella Enteritidis (95.6%; 65/68), respectively. Importantly, 96.3% (26/27) of ST34 Salmonella Typhimurium isolates possessed the ACSSuT resistance pattern (ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline). PFGE analysis of ST34 isolates showed clonal dissemination across all four types of retail foods. Our findings highlight the high prevalence of antimicrobial-resistant Salmonella isolates in retail foods in Shanghai, especially the clonal expansion of ST34 isolates with MDR-ACSSuT resistance, which might pose a public health threat.