Interaction of the plasmid-encoded quinolone resistance protein QnrB19 with Salmonella Typhimurium DNA gyrase.

BACKGROUND:Plasmid-encoded quinolone resistance protein Qnr is an important factor in bacterial resistance to quinolones. Qnr interacts with DNA gyrase and reduces susceptibility to quinolones. The gene qnr likely spreads rapidly among Enterobacteriaceae via horizontal gene transfer. Though the vast amounts of epidemiological data are available, molecular details of the contribution of QnrB19, the predominant Qnr in Salmonella spp., to the acquisition of quinolone resistance has not yet been understood well. OBJECTIVE:We aimed to examine the role of QnrB19 in quinolone resistance acquisition using recombinant Salmonella Typhimurium DNA gyrases and QnrB19. MATERIALS AND METHODS:Recombinant QnrB19 was expressed in E. coli and purified by Ni-NTA agarose column chromatography. DNA supercoiling activities of recombinant Salmonella Typhimurium DNA gyrase were assessed with or without QnrB19 under the existence of three quinolones to measure IC50s, the concentration of each quinolone required for 50% inhibition in vitro. RESULTS:The IC50s of norfloxacin, ciprofloxacin and nalidixic acid against DNA gyrases were measured to be 0.30, 0.16 and 17.7 μg/mL, respectively. The addition of QnrB19 increased the IC50s of norfloxacin and ciprofloxacin to be 0.81 and 0.48 μg/mL, respectively, where no effect of QnrB19 was observed on the IC50 of nalidixic acid. CONCLUSION:QnrB19 was shown for the first time in vitro to have ability to grant non-classical quinolone resistance to S. Typhimurium DNA gyrase. Structural insight on quinolones in this study may contribute to investigate drugs useful for preventing the spread of plasmid carrying PMQR along with other factors associating with antimicrobial resistance in S. Typhimurium and other bacteria.