In Vitro Selection oframRandsoxRMutants Overexpressing Efflux Systems by Fluoroquinolones as Well as Cefoxitin in Klebsiella pneumoniae

Abstract

ABSTRACTThe relationship between efflux system overexpression and cross-resistance to cefoxitin, quinolones, and chloramphenicol has recently been reported inKlebsiella pneumoniae. In 3 previously published clinical isolates and 17in vitromutants selected with cefoxitin or fluoroquinolones, mutations in the potential regulator genes of the AcrAB efflux pump (acrR,ramR,ramA,marR,marA,soxR,soxS, androb) were searched, and their impacts on efflux-related antibiotic cross-resistance were assessed. All mutants but 1, and 2 clinical isolates, overexpressedacrB. No mutation was detected in the regulator genes studied among the clinical isolates and 8 of the mutants. For the 9 remaining mutants, a mutation was found in theramRgene in 8 of them and in thesoxRgene in the last one, resulting in overexpression oframAandsoxS, respectively. Transformation of theramRmutants and thesoxRmutant with the wild-typeramRandsoxRgenes, respectively, abolished overexpression ofacrBandramAin theramRmutants and ofsoxSin thesoxRmutant, as well as antibiotic cross-resistance. Resistance due to efflux system overexpression was demonstrated for 4 new antibiotics: cefuroxime, cefotaxime, ceftazidime, and ertapenem. This study shows that theramRandsoxRgenes control the expression of efflux systems inK. pneumoniaeand suggests the existence of efflux pumps other than AcrAB and of other loci involved in the regulation of AcrAB expression.