Genomic evidence for interserovar transfer and evolution of blaCTX-M-15 carrying plasmid in multidrug-resistant Salmonella Enteritidis and Salmonella Virchow isolated in South Korea.

Multidrug-resistant (MDR) Salmonella enterica serotype Enteritidis (SE) carrying blaCTX-M-15 has emerged in South Korea; however, the genetic characteristics and evolutionary dynamics of the plasmids remain understudied. This study aimed to characterize the structure, diversity, and evolution of blaCTX-M-15-carrying plasmids in SE strains from human and animal sources in South Korea. Plasmid structures of blaCTX-M-15-positive SE strains from South Korea were analyzed using incompatibility groups typing and genome alignment tools. Genetic relationships and the time to most recent common ancestor (tMRCA) for the strains were determined using whole-genome SNP analysis and BEAST phylodynamic analysis. The plasmids were classified into three types (Type I-III) based on antimicrobial resistance gene profiles and incompatibility groups. Comparative genomic analysis revealed that these plasmids were cointegrates, resulting from the fusion of the SE virulence plasmid (pSENV) and pIntAMR. The pIntAMR region was identified as a chromosomally integrated form in Salmonella Virchow (SV) acquired through IS26-mediated transposition. This region contained multiple antimicrobial resistance genes and conjugative transfer elements. The tMRCA for blaCTX-M-15-carrying SE strains was estimated as June 2007, with Type III plasmids emerging around 2015, suggesting an evolutionary shift driven by selective pressure. This study demonstrates interserovar plasmid transfer between SV and SE, mediated by IS26 transposition, contributing to plasmid evolution and MDR in SE. Understanding these mechanisms is crucial for developing strategies for mitigating the spread of MDR Salmonella.