Prevalence and characterization of Salmonella serovars isolated from farm products in Shanghai

Salmonella is a major bacterial pathogen that causes foodborne illness worldwide. Farm products, including raw meat, vegetables, and dairy products, may be important vehicles for transmitting foodborne pathogens, thus representing a threat to human health. In this study, the prevalence of Salmonella isolated from farm products in Shanghai was investigated and characterized. A total of 344 samples were randomly collected from farmers' markets, supermarkets, milk factories, and a slaughterhouse in Shanghai between March 2016 and February 2017. Using the Chinese National Standard method (GB 4789.4-2010), 83 (24.1%) samples were determined to be positive for Salmonella, including 42 (42.4%) pork, 21 (41.2%) chicken, 15 (31.3%) beef, 3 (16.7%) mutton, 1 (4.5%) lettuce, and 1 (3.8%) cucumber. All egg and raw milk samples were negative for Salmonella. Ninety strains were isolated from the positive samples, and 12 serovars were identified, including Enteritidis (n = 35), Typhimurium (n = 22), Derby (n = 13), Anatum (n = 5), Agona (n = 4), Thompson (n = 3), Indiana (n = 3), Braenderup (n = 1), Heidelberg (n = 1), Infantis (n = 1), Litchfield (n = 1) and Manhattan (n = 1). All isolates were characterized by multilocus sequence typing (MLST) and subtyped by pulsed-field gel electrophoresis (PFGE). Thirteen sequence types (ST) were identified, among which ST11, ST34, and ST40 were the predominant STs. ST was highly correlated with serovar type. The results of PFGE showed that 73 PFGE patterns were generated from the isolates. In conclusion, Salmonella contamination was common among farm products in Shanghai, and the diversity of the pathogen was high among the isolates.