Prevalence and characterization of Salmonella species isolated from pigs, ducks and chickens in Sichuan Province, China

This study aimed to analyze the prevalence of Salmonella isolated from different parts of the food production chain, and to characterize these isolates. A total of 165 Salmonella enterica isolates were identified from 1382 samples taken from conventional farms, abattoirs and retail markets from 2010 to 2011 in Sichuan, China. The Salmonella isolates were assayed for serotype, antimicrobial susceptibility, prevalence of class 1 integrons and β-lactamase genes, and subtyped using pulsed-field gel electrophoresis. Among these isolates, S. enterica serotypes Derby (76 isolates, 46%) and Typhimurium (16 isolates, 10%) were the most prevalent, and high antimicrobial resistance rates were observed for tetracycline (77%), sulfamethoxazole/trimethoprim (43%), nalidixic acid (41%) and spectinomycin (41%). Class 1 integrons were detected in 21% of these isolates, and contained gene cassettes dfrA12–aadA2, dfrA1–aadA1, dfrA1, blaPSE-1 and dfrA1/aadA2. blaOXA-1 was the most commonly identified β-lactamase gene (n = 14), followed by blaTEM-1 (n = 6), blaPSE-1 (n = 4) and blaCMY-2 (n = 1). A S. enterica serotype Indiana isolate derived from chicken from a market was positive for both blaOXA-1 and blaCMY-2, and resistant to nine tested antibiotics. The PFGE patterns were diverse. Our findings indicated that most isolates from different sampling sites were phenotypically and genetically diverse, and Salmonella was widespread and may transmit along the food production chain from farm to market. Isolates with decreased susceptibility to fluoroquinolones and extended-spectrum cephalosporins, which are used to fight foodborne Salmonella, pose a serious threat to public health.