Characterization of multidrug-resistant Salmonella Typhimurium isolates from Korean swine farms and plasmids harboring fluoroquinolone and third-generation cephalosporin resistance genes

Salmonella enterica serovar Typhimurium is a major foodborne pathogen, and the overuse of antimicrobials has contributed to the emergence of multidrug-resistant (MDR) strains. Resistance to third-generation cephalosporins and fluoroquinolones is of particular concern because of their critical importance in human medicine. In this study, we characterized the phenotypic and genotypic features of MDR S. Typhimurium isolates from pig farms using whole-genome sequencing (WGS). Among 45 S. Typhimurium isolates recovered from 129 pig farms, 29 were identified as MDR based on phenotypic screening and subjected to WGS. Phenotypically, the MDR isolates exhibited high resistance to aminoglycosides (100%), penicillins (97%), and phenicols (90%). WGS identified five isolates coharboring the third-generation cephalosporin resistance gene bla CTX-M-65 and the fluoroquinolone resistance gene qnrS1 , whereas four additional isolates carried qnrS1 alone. The bla CTX-M-65 gene was detected exclusively on IncHI2-type plasmids, whereas qnrS1 was detected on IncHI2-, IncF-type, and unclassified plasmids. In the analyzed plasmids, bla CTX-M-65 was located between mobQ–fipA and IS 903 B , whereas qnrS1 was flanked by IS 15 DIV–transposase and hin–ISKpn 19. These findings improve our understanding of the phenotypic resistance profiles, resistance genes, and plasmids associated with MDR S. Typhimurium from pig farms and support continued genomic surveillance for monitoring the dissemination of clinically important resistance determinants.