Antimicrobial Resistance and Genetic Characteristics of Ciprofloxacin-Resistant Escherichia coli from Suckling and Weaned Piglet Housing Areas in Korea

In swine production systems, antimicrobial exposure and management conditions vary across production stages, particularly during the transition from suckling to weaning, which may influence the selection and persistence of antimicrobial-resistant bacteria. We compared the antimicrobial resistance patterns of 70 Escherichia coli isolates obtained from suckling and weaned piglet housing areas (35 isolates each) on 35 farms in Korea and further characterized the genetic features of the 40 ciprofloxacin (CIP)-resistant isolates. Isolates showed high rates of resistance to several antimicrobials, including streptomycin (88.6%) and ampicillin (85.7%). In an exploratory comparison, CIP resistance was higher in isolates from weaned piglet housing areas (71.4%) than in those from suckling piglet housing areas (42.9%; unadjusted two-sided exact p = 0.04139, McNemar's test). Mutations in the quinolone resistance-determining regions (QRDRs) were detected in 27 of the 40 CIP-resistant isolates (67.5%), predominantly gyrA S83L/D87N and parC S80I substitutions. Plasmid-mediated quinolone resistance (PMQR) determinants were detected in nine isolates; qnrD was the most common gene (6/9, 66.7%). Multilocus sequence typing analysis of the nine PMQR-positive isolates identified multiple sequence types; ST5229 was the most common sequence type (5/9, 55.6%) and was detected in both housing-area groups. The higher CIP resistance observed in isolates from weaned piglet housing areas indicates a production-stage-associated difference in FQ resistance during early pig production.