Plasmid-encoded multidrug efflux pump conferring resistance to olaquindox in Escherichia coli

We report here the first gene-encoded resistance mechanism to the swine growth enhancer olaquindox. The genetic elements involved in resistance to olaquindox were subcloned and sequenced from a conjugative plasmid isolated from Escherichia coli. The subcloned fragment contained two open reading frames, oqxA and oqxB, that are homologous to several resistance-nodulation-cell-division family efflux systems from different species. The putative protein sequences were aligned to both experimentally verified and putative efflux pumps. We show that oqxA and oqxB are expressed in E. coli. Plasmids containing the oqxAB genes yielded high (>128 microg/ml) resistance to olaquindox in E. coli, whereas strains containing the control plasmid showed low resistance to the drug (8 microg/ml). The oqxAB-encoded pump also conferred high (>64 microg/ml) resistance to chloramphenicol. We demonstrate that the subcloned fragment conferred H(+)-dependent ethidium efflux abilities to E. coli strain N43. In addition, we show that the efflux system is dependent on the host TolC outer membrane protein when expressed in E. coli.