pHN7A8 is a chimera that may have resulted from the acquisition, by recombination in the plasmid backbone, of the multiresistance region found in pXZ. This region appears to have evolved from the resistance determinant R100 through the stepwise integration of multiple antimicrobial resistance determinants from different sources by the actions of mobile elements and recombination. The successful dissemination of this multidrug resistance plasmid presents further challenges for the prevention and treatment of Enterobacteriaceae infections.
Fifty-one fluoroquinolone-resistant Escherichia coli isolates recovered from pigs, workers and environmental samples in one pig farm were screened for 16S rRNA methylase genes and qepA, a fluoroquinolone efflux pump gene, by PCR. Clonal relatedness of the E. coli isolates was examined by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing and phylogenetic analysis. Plasmids from the E. coli isolates were characterized by incompatibility group, restriction enzyme digestion and Southern hybridization analysis. The genetic environment of rmtB and qepA was also determined by PCR mapping. Eleven isolates that were highly resistant to amikacin and fluoroquinolones were positive for rmtB and qepA. All of these isolates belonged to phylogenetic group A, but most of them had different PFGE patterns or belonged to different sequence types (STs). Four isolates from different sources (two from pigs, one from a farm worker and one from an environmental sample) belonged to the same ST (ST160). Both rmtB and qepA were located on approximately 75-kb IncFII conjugative plasmids with nearly the same EcoRI digestion pattern. Tn3, IS26 and ISCR3 were found to be associated with rmtB and qepA. This study has found, for the first time, the transmission of rmtB and qepA among E. coli isolates from pigs, farm workers and the environment. Both horizontal transfer of IncFII plasmids and clonal dissemination have occurred and been seen to contribute to the dissemination of these resistance genes in a pig farm.
Both horizontal gene transfer and clonal spread could be responsible for the dissemination of the rmtB gene in the pig farm and its environment. To our knowledge, this study is the first report of rmtB-positive bacteria from farmland soils and indicates that these antibiotic-resistant bacteria and/or resistance genes could be acquired by humans through the food chain.
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