Molecular epidemiology of pESI-carrying <i>Salmonella</i> Infantis in Korea: insights from a one health framework

The emergence and global dissemination of <i>Salmonella</i> Infantis carrying the pESI megaplasmid is a growing public health concern owing to its association with multidrug resistance (MDR), enhanced virulence, and increased environmental adaptability. This study investigated the molecular epidemiology and transmission dynamics of pESI-carrying <i>S.</i> Infantis in the poultry production chain and human clinical cases in Korea. A total of 295 <i>Salmonella</i> isolates were collected from poultry sources (farms, slaughterhouses, and retail meats) in 2023, of which 219 (74.2%) were identified as <i>S.</i> Infantis. Additionally, 22 human-origin <i>S</i>. Infantis isolates collected between 2016 and 2023 were included in this study. Among these 241 <i>S</i>. Infantis isolates, 218 from poultry and one from a human case (collected in 2023) were MDR and carried pESIs, most of which carried <i>blaCTX-M-65</i> and multiple other resistance genes. Comparative whole-genome sequencing revealed a high degree of genetic similarity between poultry and human pESI-carrying <i>S.</i> Infantis isolates, suggesting probable zoonotic transmission through the chicken supply chain. Structural comparison of complete plasmid sequences further revealed close similarity between Korean and US pESIs, suggesting a potential epidemiological link. This study identified the rapid clonal spread of pESI-carrying <i>S</i>. Infantis in the Korean poultry sector and reported the first identification of pESI-carrying <i>S</i>. Infantis in human clinical isolates in Korea. These findings underscore the urgent need for global surveillance, enhanced biosecurity measures in the poultry industry, and stricter regulations to prevent further dissemination of MDR <i>S</i>. Infantis.