In regard to genetic characterization of resistance, only alterations in gyrA were found for levofloxacin, however, alterations in gyrA and parC were found for ciprofloxacin and prulifloxacin. Point mutations within DNA gyrase are known to cause a reduction in the affinity of the enzyme for FQs, decreasing the susceptibility of bacteria to these molecules. Topoisomerase IV is the

second target for FQ in the absence of susceptible gyrase. Therefore, multiple mutations in gyrA and/or parC are required for high level FQ resistance in E. coli [23, 24]. In our study, both ciprofloxacin and prulifloxacin resistant mutants presented mutations in gyrA and parC, while levofloxacin resistance was found associated only with mutations in gyrA. These results seem to indicate click here that levofloxacin resistance at a concentration observed during treatment might VX-680 develop more slowly and might be lower than resistance to the other FQs tested in the present study. However, this study did not evaluated other mechanisms other than the target enzyme that

might be involved in the observed resistant strains, including decreased intracellular drug accumulation as a result of alterations in the outer membrane proteins of the wall cell, or active efflux of the drug mediated by a number of efflux pumps. As far as FQ resistance in Klebsiella spp. is concerned, plasmid-mediated quinolone resistance mechanisms associated with the qnr gene and the aac(6′)-Ib-cr gene in ESBL producing strains have been described [25, 26]. The first encodes target protection proteins of the pent peptide repeat family and seems to be associated with low level quinolone STK38 resistance, while the aac(6′)-Ib-cr gene encodes a variant of the

common aminoglycoside acetyltransferase which is able to reduce the activity of some FQ, thus enhancing the selection of chromosomal mutations [25]. Although in the present study the presence of plasmid-mediated resistance was not investigated, it can not be excluded that these genes might be involved in selection of resistance observed after serial exposure to fluoroquinolones. In a previous study, we have shown that combinations of a fluoroquinolone with a beta-lactam may both provide improved antimicrobial activity and limit the occurrence of resistance in ESBL-producing E. coli clinical isolates [27]. Therefore, the use of combination therapy could be an attractive strategy to limit occurrence of resistance. Conclusions In conclusion, among the tested fluoroquinolones, levofloxacin was the most able to limit occurrence of resistance in vitro. However, in order to limit the occurrence of resistance, appropriate dosages of fluoroquinolones should be respected in the therapy of infections caused by Enterobacteriaceae, as well as use of synergistic combinations in the most complicated infections. Methods Strains Twenty clinical isolates of E. coli and Klebsiella spp.