Salmonella Typhimurium Isolated from Healthy Pigs and Their Ability of Horizontal Transfer of Multidrug Resistance and Virulence Genes

Abstract

Salmonella is the most common food-borne pathogen worldwide. Rapid dissemination of multidrug-resistant strains, in particular from animal origins, is a growing concern to human and animal health, and it is mostly attributed to conjugative DNA exchange in the intestinal tract of food animals. To understand the potential role of pigs as a reservoir for antimicrobial-resistant Salmonella, we isolated Salmonella from slaughtered pigs and examined their drug resistance and gene transfer ability. We collected fecal and carcass swabs from 104 healthy pigs at an abattoir in Obihiro, Hokkaido, Japan. A total of 15 Salmonella strains were isolated, the most common serotypes being S. Typhimurium (7/15), S. Derby (2/15), S. Southampton (2/15) and S. 04:d:-(4/15). All of S. Typhimurium isolates were resistant to at least one of five antibiotics (ampicillin, kanamycin, sulfisoxazole, tetracycline and streptomycin). Pulsed-field gel electrophoresis (PFGE) profiles after Xbal and BlnI digestion were analyzed. S. Typhimurium isolates from 3 farms located in different regions clustered together and showed genetic relatedness. In conjugation experiments, one multidrug-resistant S. Typhimurium isolate showed the ability to transfer not only antibiotic resistance genes but also virulence genes such as spvABC to recipient bacteria. These results suggest that the spread of S. Typhimurium had occured in pig farms and that asymptomatic Salmonella-infected pigs should be considered as a significant source of antibiotic-resistant bacteria.