Genomic characterization of multidrug-resistant Salmonella Choleraesuis ST145 isolated from a pig in South Korea and its phylogenomic relationship to global lineages.

Salmonella Choleraesuis is a highly invasive zoonotic pathogen causing systemic infections in swine and humans. While rare in South Korea, an ST145 Kunzendorf variant was isolated from healthy pig feces through the national surveillance system. We performed whole-genome sequencing (WGS) to elucidate its resistance and virulence mechanisms. The WGS showed a genome depth of 687.76×, with 5,014,689 bp in size, GC (52.1%), N50 (4,732,700 bp), and completeness (100%). The isolate exhibited a multidrug-resistant (MDR) phenotype. WGS revealed two key plasmids: an IncHI2 MDR reservoir (232 kb) carrying genes such as blaTEM-1, tet(A), aac(3)-IVa, aph(4)-Ia, mph(B), and floR; and a pSCV50-like virulence plasmid (50 kb) harboring the spvRABCD and pefABCD operons. The core-genome multi-locus sequence typing (cgMLST) showed that the isolate is nearly identical to a 2020 domestic strain (HC5 level). Furthermore, its phylogenetic proximity suggests genetic similarity to human-derived isolates from France (HC20 level). Taken together, these findings highlight the persistent circulation of potentially virulent MDR clones and the necessity for rigorous genomic surveillance to prevent zoonotic outbreaks.