Evaluation of the contribution of gyrA mutation and efflux pumps to fluoroquinolone and multidrug resistance in pathogenic Escherichia coli isolates from dogs and cats

Objective-To investigate the contribution of gyrA mutation and efflux pumps to fluoroquinolone resistance and multidrug resistance among Escherichia coli isolates from dogs and cats Sample Population-536 clinical isolates of E coli Procedures-Minimum inhibitory concentrations (MICs) were determined for enrofloxacir and 6 other drug classes by use of broth microdilution techniques Real-time PCR assay was used to determine the mutation in gyrA Phe-Arg naphthylamide an efflux pump inhibitor was used to examine the contribution of efflux pump overexpression Results-The MIC for fluoroquinolones increased in a stepwise fashion and was lowest in the absence of mutations higher with a single point mutation and highest with 2 point mutations Level of resistance in the latter category was high (8 times the breakpoint) but this was associated with expression of the AcrAB efflux pump Inhibition of the efflux pump resulted in a reduction in the MIC to less than the susceptible breakpoint for isolates with an MIC <= 4 mg/L regardless of the presence of a mutation The greatest magnitude in MIC decrease (MIC was decreased by a factor of > 67 fold) was for isolates with a single mutation but the greatest absolute decrease in MIC (124 mg/L) was for isolates with 2 mutations Inhibition of the AcrAB efflux pump in isolates characterized by multidrug resistance decreased the MIC of drugs structurally unrelated to fluoroquinolone Conclusions and Clinical Relevance-Fluoroquinolone resistance in E appearec to be a stepwise phenomenon with MIC increasing as the number of point mutations in gyrA increased but high level resistance and multidrug resistance a,sociated with fluoioquinolone resistance reflected overexpression of the AcrAB efflux pump (Am J Ve Res 2011 7225-32)