Evolutionary trends and genetic determinants of third-generation cephalosporin resistance in Escherichia coli from Korean livestock: a 14-year longitudinal study, 2010-2023.

The emergence of third-generation cephalosporin-resistant Escherichia coli in livestock is a critical public health concern due to its potential transmission to humans via the food chain. This study investigated the 14-year longitudinal trends (2010-2023) of third-generation cephalosporin resistance, its genetic determinants, and phenotypic characteristics in E. coli isolated from various livestock species in South Korea. Resistance was significantly higher in chickens and pigs than in cattle, with a marked increase observed in healthy chickens over the last decade. A major shift in extended-spectrum beta-lactamase (ESBL) genotypes occurred around 2014, where bla CTX - M-55 emerged as the predominant variant (52.3%, 112/214), surpassing bla CTX - M-14 and bla CTX - M-15. The majority of isolates (88.8%, 190/214) exhibited multidrug resistance (MDR), frequently involving co-resistance to fluoroquinolones and colistin. These resistance determinants were highly transferable, primarily mediated by IncF and IncI1 plasmids, associated with mobile genetic elements such as ISEcp1 and IS26. Multi-locus sequence typing (MLST) revealed high genetic diversity, with ST752, ST457, and ST10 identified as the most prevalent lineages. Next-generation sequencing (NGS) further confirmed the presence of megaplasmids co-harboring ESBL genes, mcr-1, metal resistance genes, and diverse virulence factors. These findings highlight a significant evolutionary shift toward bla CTX - M-55 dominance and the rising threat of MDR E. coli in Korean livestock. The high transferability and virulence of these resistance determinants underscore the urgent need for continuous "One Health" surveillance and integrated antimicrobial stewardship to mitigate the risk of zoonotic transmission.