Enhanced Expression of the Multidrug Efflux Pumps AcrAB and AcrEF Associated with Insertion Element Transposition in Escherichia coliMutants Selected with a Fluoroquinolone

ABSTRACTThe development of fluoroquinolone resistance in Escherichia colimay be associated with mutations in regulatory gene loci such as marRABthat lead to increased multidrug efflux, presumably through activation of expression of the AcrAB multidrug efflux pump. We found that multidrug-resistant (MDR) phenotypes with enhanced efflux can also be selected by fluoroquinolones frommarRAB- or acrAB-inactivated E. coliK-12 strains having a single mutation in the quinolone-resistance-determining region of gyrA. Mutant 3-AG100MKX, obtained from a marknockout strain after two selection steps, showed enhanced expression of acrBin a reverse transcriptase PCR associated with insertion of IS186into the AcrAB repressor gene acrR. In vitro selection experiments with acrABknockout strains yielded MDR mutants after a single step. Enhanced efflux in these mutants was due to increased expression of acrEFand associated with insertion of IS2 into the upstream region ofacrEF, presumably creating a hybrid promoter. These observations confirm the importance of efflux-associated nontarget gene mutations and indicate that transposition of genetic elements may have a role in the development of fluoroquinolone resistance in E. coli.