This study examined the phenotypic and molecular characteristics of FQ-resistant E. coli from different pig production stages (suckling, weaned, grower, finisher, and sow). Antimicrobial susceptibility, minimum inhibitory concentrations, quinolone resistance determinants, plasmid-mediated quinolone resistance (PMQR) genes, and multilocus sequence types (STs) were analyzed in 104 E. coli isolates collected from pig farms in Korea. To facilitate a systematic comparison, resistance was evaluated in a stage-stratified manner and integrated with molecular typing and clonal analysis. FQ resistance prevalence was high (28.6–66.7%) across all pig production stages. PMQR genes (predominantly qnrD and qnrS) were detected in 37.8% of isolates; all exhibited multidrug resistance (11–18 antimicrobials), and 41.2% possessed class I integrons. Over half of the FQ-resistant isolates (25 isolates) had enrofloxacin MICs > 256 μg/mL. ciprofloxacin MICs ranged from 8–256 μg/mL, while NOR resistance was consistently high, with MICs of 256 μg/mL. Ten distinct STs were identified, with ST5229 being the most prevalent (41.2%) across multiple production stages. Phylogenetic analysis revealed three clades: clade A (primarily ST5229) spanned multiple production stages, which indicates vertical transmission, while clades B and C indicated horizontal gene transfer. These findings emphasize the significance of integrated surveillance and responsible antimicrobial stewardship in mitigating the spread of FQ-resistant E. coli in pig production.

To Cite This Article: Do KH, Seo KW and Lee WK 2025. Genetic and phylogenetic characterization of fluoroquinolone-resistant Escherichia coli from farrowing to finishing in Korean pig farms. Pak Vet J. http://dx.doi.org/10.29261/pakvetj/2025.249