Enhanced expression of the multidrug efflux pumps AcrAB and AcrEF associated with insertion element transposition in Escherichia coli mutants selected with a fluoroquinolone

The development of fluoroquinolone resistance in Escherichia coli may be as sociated with mutations in regulatory gene loci such as marRAB that lead to increased multidrug efflux, presumably through activation of expression of the AcrAB multidrug efflux pump, We found that multidrug-resistant (MDR) p henotypes with enhanced efflux can also be selected by fluoroquinolones fro m marRAB- or acrAB-inactivated E. coli K-12 strains having a single mutatio n in the quinolone-resistance-determining region of gyrA, Mutant 3-AG100MKX , obtained from a mar knockout strain after two selection steps, showed enh anced expression of acrB in a reverse transcriptase PCR associated with ins ertion of IS186 into the AcrA8 repressor gene acrR, In vitro selection expe riments with acrAB knockout strains yielded MDR mutants after a single step . Enhanced efflux in these mutants was due to increased expression of acrEF and associated with insertion of IS2 into the upstream region of acrEF, pr esumably creating a hybrid promoter. These observations confirm the importa nce of efflux-associated nontarget gene mutations and indicate that transpo sition of genetic elements may have a role in the development of fluoroquin olone resistance in E. coli.