Temporal trends in Salmonella serovar distribution and antimicrobial resistance profiles, including plasmid-mediated colistin resistance, among livestock-derived isolates in South Korea, 2019–2024

Amid the global threat of emerging antimicrobial resistance, the transmission of multidrug-resistant (MDR) Salmonella from livestock remains a critical concern for public health. This study investigated the longitudinal trends of Salmonella serovars and the molecular epidemiology of colistin resistance in 4,198 isolates from cattle, pigs, chickens, and ducks in South Korea (2019–2024). Among the 93 serovars identified, S . Infantis (40.1% in chickens), S . Typhimurium and variant (49.8% in pigs), and S . Typhimurium (39.1% in ducks) were predominant, with S . Infantis showing a dramatic surge in poultry during 2022–2024. Overall, 78.7% of isolates were resistant to at least one antimicrobial, and 50.5% were MDR; S . Infantis exhibited a particularly high MDR rate (90.7%), including significant resistance to the high-priority critically important antimicrobial cefotaxime (85.3%). Molecular characterization of colistin-resistant isolates revealed the presence of the mobile colistin resistance gene, mcr-1 , localized on highly transmissible IncX4 and IncI2 plasmids in S . Infantis and S . Typhimurium, respectively. Comparative genomics via next-generation sequencing (NGS) showed over 99% homology between these plasmids and previously reported human clinical plasmids, and their horizontal transferability was confirmed through conjugation assays. These findings highlight a critical epidemiological shift toward hyper-resistant S . Infantis and the persistence of mobile mcr-1 within the livestock reservoir, posing a substantial risk of zoonotic transmission through the food chain. This study underscores the urgent need for enhanced global surveillance and stricter antimicrobial stewardship to mitigate the dissemination of last-resort resistance at the animal-human interface.