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C. M. Sanfilippo, W. Haas, C. K. Hesje, T. W. Morris, D. L. Shinabarger; The Effect of Bacterial Efflux Pumps on Susceptibility to Besifloxacin and Other Ophthalmic Fluoroquinolones. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5720.
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High-level resistance to fluoroquinolones (FQs) is generally mediated by point mutations in the genes encoding DNA gyrase and topoisomerase IV, while energy-dependent efflux pumps contribute usually to a lesser degree. Here, we determined the effect of efflux pumps on the minimum inhibitory concentration (MIC) values for besifloxacin, a novel ophthalmic FQ.
MICs were determined for five species of wild-type (n=14) and mutant strains (n=14) that overexpress or lack known efflux pump genes, such as norA, pmrA, or acrAB. Test and control agents included besifloxacin (BES), sparfloxacin (SPX), ciprofloxacin (CIP), norfloxacin (NOR), moxifloxacin (MXF), tetracycline (TET), and ethidium bromide (EtBr). Efflux pump inhibitors included reserpine, carbonyl cyanide m-chlorophenyl-hydrazone, and sodium orthovanadate.
Efflux pump inhibitors had no or only little effect on MIC values for BES, SPX, MXF, or TET. In contrast, a decrease of up to 16-fold in MIC values was seen for CIP, NOR, and EtBr in the presence of reserpine.In S. aureus, norA overexpression resulted in a 2× increase in MIC values for SPX and MXF, 4× for BES, 32× for NOR, and 64× for CIP and EtBr. Addition of reserpine reduced those MIC values, especially for NOR, CIP, and EtBr. Mutations in the pmrA or patA genes of S. pneumoniae had no or little effect on BES, SPX and MXF, but changed the MIC values for CIP, NOR, TET, and EtBr by 4- to 8-fold.Deletion of acrAb in H. influenzae resulted in an 8× decrease in EtBr MIC values, but had no effect on any of the other test agents. In contrast, inactivation of acrAB or tolC in E. coli reduced the MIC values for the FQs by 2- to 16-fold and EtBr by ≥64-fold. Inactivation of oprM in P. aeruginosa decreased MIC values for all FQs by 16-fold and TET by 32-fold.
SPX, MXF, and BES are poor substrates for NorA in S. aureus and PmrA or PatAB in S. pneumoniae, while NOR, CIP, and EtBr are good substrates. The AcrAB pump does not contribute to FQ resistance in H. influenzae, but does play a role in E. coli. In P. aeruginosa, OprM is involved in FQ efflux, independent of the structure of the FQ. Overall, the effect of efflux pumps on BES MIC values was similar to that of MXF.
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