Prevalence, Antimicrobial Resistance, and Relatedness of Salmonella Isolated from Chickens and Pigs on Farms, Abattoirs, and Markets in Sichuan Province, China

This study aims at investigating the distribution, antimicrobial resistance, and genetic relationship of Salmonella isolated from 18 farms, their downstream abattoirs, and markets of chickens and pigs in Sichuan province, China. A total of 193 Salmonella isolates were identified from 693 samples with an isolation rate of 26.27% (88/335) in chickens and 29.33% (105/358) in pigs. Salmonella was isolated more frequently in abattoirs and markets than from farms. Serotypes were determined according to the White-Kauffmann-Le Minor scheme and 16 different serotypes were identified, with Derby being the most common, followed by Typhimurium and Meleagridis. Antimicrobial resistance phenotypes and genotypes were studied by using the disk diffusion method and polymerase chain reaction (PCR) amplification, respectively. Overall, 44.04% (n=85) of all isolates were multidrug resistant (MDR) and resistance to nalidixic acid (51.30%) was the most frequently observed. bla(CTX-M-55) was the most prevalent extended-spectrum -lactamases gene, and polymyxin resistance gene mcr-1 was present in strains with various serotypes. Multilocus sequence typing indicated that sequence type (ST) had a close relationship with serotype, and 34.20% of all strains were ST40, which was the most prevalent. The unweighted pair group method with arithmetic means (UPGMA) dendrogram of pulsed-field gel electrophoresis showed that Salmonella isolates belonging to the same serovar from different parts of the production chain were highly genetic related, indicating that Salmonella as well as resistance genes could potentially be transmitted from farms to markets. Our study highlights the fact that Salmonella isolates from chicken and pig production chain were frequently exhibiting MDR profiles, and the dissemination of MDR Salmonella from farm to market could pose significant threats to food safety and public health.