An EmrB multidrug efflux pump in with unexpected roles in antibiotic resistance.

The antibiotic trimethoprim is frequently used to manage infections, and members of the resistance-nodulation-division (RND) family of efflux pumps have been implicated in multidrug resistance of this species complex. We show here that a member of the distinct multidrug resistance B (EmrB) family is a primary exporter of trimethoprim in , as evidenced by increased trimethoprim sensitivity after inactivation of , the gene that encodes EmrB. We also found that the gene is up-regulated following the addition of gentamicin and that this up-regulation is due to repression of the gene encoding OstR, a member of the multiple antibiotic resistance regulator (MarR) family. The addition of the oxidants HO and CuCl to cultures resulted in OstR-dependent differential expression, as determined by qRT-PCR analysis. Specifically, OstR functions as a rheostat that optimizes expression under oxidizing conditions, and it senses oxidants by a unique mechanism involving two vicinal cysteines and one distant cysteine (Cys, Cys, and Cys) per monomer. Paradoxically, inactivation increased resistance of to tetracycline, a phenomenon that correlated with up-regulation of an RND efflux pump. These observations highlight the intricate mechanisms by which expression of genes that encode efflux pumps is optimized depending on cellular concentrations of antibiotics and oxidants.