Recent Occurrence and Rapid Spread of Multidrug-Resistant Salmonella Infantis in Broiler Flocks in Korea.

Salmonella Infantis has recently been one of the most prevalent serotypes in poultry and has been identified in human salmonellosis cases worldwide. Multidrug-resistant (MDR) Salmonella Infantis has emerged as a significant threat to both poultry production and public health due to its increasing prevalence and global dissemination. We identified the occurrence of an MDR Salmonella Infantis clone in broiler flocks in Korea, and the clone was characterized to explore potential genetic causes for its high prevalence and rapid spread in broiler production. In total, 220 Salmonella strains isolated between 2020 and 2023 from broiler flocks were serotyped, and 50 strains were identified as Salmonella Infantis (22.7%). The isolates were tested for antimicrobial susceptibility, and their genetic characteristics were analyzed using pulsed-field gel electrophoresis (PFGE), multilocus sequence typing, and whole genome sequencing (WGS). Forty-six strains of Salmonella Infantis isolated since 2020 were resistant to at least five antimicrobial families including ampicillin, cephalosporins, chloramphenicol, nalidixic acid, and tetracycline. The strains showed 10 PFGE patterns and a single multilocus sequence type 32. Eight representative MDR strains were analyzed by WGS. Seven of the eight strains carried the plasmid of emerging Salmonella Infantis-like megaplasmids recognized globally in emergent MDR Salmonella Infantis. They had a high prevalence of seven antimicrobial resistance genes, six of which were identified in plasmids. Also, they all share virulence genes, including fimbrial adherence determinants and secretion system components, and showed a clonal relationship to strains from North America, South America, and West Asia, suggesting potential international dissemination routes. To mitigate the risks associated with the rapid spread of MDR Salmonella Infantis in poultry production and its potential impact on human health, this study provides valuable insights into implementing effective control measures to reduce Salmonella in broiler production in Korea. Further highlighting the critical importance of enhanced biosecurity and continuous surveillance.