Occurrence and sequence type of antimicrobial resistant Salmonella spp. circulating in antibiotic-free organic pig farms of northern-Thailand

Salmonella is one of the major foodborne pathogens, often as the result of pork consumption. Currently, with the situation of antimicrobial resistance, organic farming has been suggested as an alternative for healthier options. However, there is little evidence to support this. In this study, we investigated the occurrence of Salmonella circulating in local organic pig farms in northern Thailand and typed isolated clones to better understand the population structure and transmission dynamics of the underlying Salmonella contamination. In total, 112 samples from 11 organic pig farms were processed from October to December 2018. Salmonella was detected in 9 targeted farms. One-fourth (28/112) of all samples yield Salmonella. The positives proportion of fecal, feeder swabs, and boot swabs were found to be 32.7% (17/53), 17.7% (6/34), and 20.0% (5/25), respectively. Of the 28 positive strains, Seven Salmonella serotypes were identified, with S. Rissen being the most common (15/28; 53.6%). 89.3% (25/28), 78.6% (22/28) and 71.4% (20/28) of isolated Salmonella resisted against tetracycline, ampicillin, and sulfamethoxazole-trimethoprim, respectively. From multilocus sequence typing (MLST) analysis, the phylogenetic tree hinted that cross contamination within herds, point mutation of the housekeeping genes at period persisted in a herd, and sharing routes of supply chain between farms. A minimum spanning tree (MST) revealed that Salmonella contamination in organic pig farming is possibly linked with conventional farming. Based on the own results, strictly highly organic practices provide a safe alternative enhancing domestic consumer trust and improve public health safety.