Prevalence of Plasmid-Mediated Quinolone Resistance Determinants Among Escherichia coli Isolated from Food Animals in Korea

The aim of this study was to investigate the prevalence of plasmid-mediated quinolone resistance (PMQR) determinants in Escherichia coli isolated from food-producing animals and to characterize the PMQR-positive isolates. A total of 365 E. coli isolates which were either nalidixic acid resistant and ciprofloxacin susceptible (NAL(R)-CIPS; n = 185), or nalidixic acid and ciprofloxacin resistant (NAL(R)-CIPR; n = 180) were assessed for the presence of PMQR determinants by polymerase chain reaction. PMQR-positive isolates were further characterized by mutation analysis within the quinolone resistance-determining region (QRDR) of gyrA, gyrB, parC, and parE, phylogenetic group analysis, and pulsed-field gel electrophoresis (PFGE). Fourteen NAL(R)-CIPS (n = 8) and NAL(R)-CIPR (n = 6) E. coli isolates were positive for PMQR genes. Among them, qnrB4, qnrS1, and aac(6')-Ib-cr genes were detected in two (0.5%), eight (2.2%), and four (1.1%) isolates, respectively. None of the isolates harbored qnrA, qnrC, qnrD, and qepA genes. All but one PMQR-positive isolates harbored one or more point mutations in the QRDR of gyrA, and five of these isolates had additional mutations in the parC gene. Furthermore, one isolate each had additional substitutions in gyrB and parE genes, respectively. The most prevalent mutation was Ser83-Leu within the QRDR of gyrA. Phylogenetic analysis identified three major phylogenetic lineages, with phylogroups A (n = 7) and D (n = 4) being the most common phylogroups. None of the isolates belonged to virulent phylogroup B2. PFGE demonstrated that a combination of clonal and horizontal gene transmission is disseminating PMQR genes among the veterinary E. coli isolates in Korea. To our knowledge, this is the first report of occurrence of qnrB, qnrS, and aac(6')-Ib-cr genes in E. coli isolated from food-producing animals in Korea. Isolation of PMQR genes from food animals is a matter of concern since they could be transmitted to humans via food animals.