Dissemination and Characterization of Plasmids Carrying oqxAB-bla (CTX-M) Genes in Escherichia coli Isolates from Food-Producing Animals

BACKGROUND: The association of PMQR and ESBLs in negative-bacteria isolates has been of great concern. The present study was performed to investigate the prevalence of co-transferability of oqxAB and bla (CTX-M) genes among the 696 Escherichia coli (E. coli) isolates from food-producing animals in South China, and to characterize these plasmids. METHODS: The ESBL-encoding genes (bla (CTX-M), bla (TEM) and bla (SHV)), and PMQR (qnrA, qnrB, qnrS, qnrC, qnrD, aac(6’)-Ib-cr, qepA, and oqxAB) of these 696 isolates were determined by PCR and sequenced directionally. Conjugation, S1 nuclease pulsed-field gel electrophoresis (PFGE) and Southern blotting experiments were performed to investigate the co-transferability and location of oqxAB and bla (CTX-M). The EcoRI digestion profiles of the plasmids with oqxAB-bla (CTX-M) were also analyzed. The clonal relatedness was investigated by PFGE. RESULTS: Of the 696 isolates, 429 harbored at least one PMQR gene, with oqxAB (328) being the most common type; 191 carried bla (CTX-M), with bla (CTX-M-14) the most common. We observed a significant higher prevalence of bla (CTX-M) among the oqxAB-positive isolates (38.7%) than that (17.4%) in the oqxAB-negative isolates. Co-transferability of oqxAB and bla (CTX-M) was found in 18 of the 127 isolates carrying oqxAB-bla (CTX-M). These two genes were located on the same plasmid in all the 18 isolates, with floR being on these plasmids in 13 isolates. The co-dissemination of these genes was mainly mediated by F33:A-: B- and HI2 plasmids with highly similar EcoRI digestion profiles. Diverse PFGE patterns indicated the high prevalence of oqxAB was not caused by clonal dissemination. CONCLUSION: bla (CTX-M) was highly prevalent among the oqxAB-positive isolates. The co-dissemination of oqxAB-bla (CTX-M) genes in E. coli isolates from food-producing animals is mediated mainly by similar F33:A-: B- and HI2 plasmids. This is the first report of the co-existence of oqxAB, bla (CTX-M), and floR on the same plasmids in E. coli.