Evolution and Dissemination of OqxAB-Like Efflux Pumps, an Emerging Quinolone Resistance Determinant among Members of Enterobacteriaceae
ABSTRACTThe OqxAB efflux pump, a plasmid-mediated quinolone resistance (PMQR) determinant, has become increasingly prevalent among members ofEnterobacteriaceaeover the past decade. To investigate the evolution and dissemination routes of theoqxABoperon, we assessed the prevalence ofoqxAB-like elements among various Gram-negative bacterial species and analyzed the genotypic and phenotypic characteristics of organisms harboring such elements. With a comprehensive genotyping approach, a chromosome-basedoqxABoperon was detectable in allKlebsiella pneumoniaestrains tested, including organisms isolated before the year 1984. Sequence and phylogenetic analyses confirmed that theoqxABoperon inK. pneumoniaeisolates was genetically closest to their plasmid-borne counterparts recoverable only fromEscherichia coliandSalmonellaisolates collected from the year 2003 onward. Chromosomal elements with much lower sequence homology were also found among theEnterobacterspp. but not other Gram-negative species. Contrary to the quinolone resistance phenotypes which were consistently observable among organisms withoqxAB-harboring plasmids, chromosomaloqxABelements generally did not confer quinolone resistance, except forK. pneumoniaestrains, which exhibited a typicaloqxAB-mediated phenotype characterized by cross-resistance to olaquindox, chloramphenicol, and the quinolones. Gene expression analysis illustrated that such phenotypes were due to elevated expression of the chromosomaloqxABoperon. Furthermore, transposition of theoqxABoperon from the bacterial chromosome to plasmids was found to result in a >80-fold increase in the level of expression of the OqxAB pump, confirming its status as the first constitutively expressed efflux system located in bacterial mobile elements.