Overexpression of resistance-nodulation-division (RND)-type efflux pumps is an important mechanism for bacteria to combat antimicrobials. RND efflux pumps are also critical for bacterial physiology such as oxidative stress tolerance. Stenotrophomonas maltophilia, a multi-drug resistant opportunistic pathogen, harbors eight RND-type efflux pump operons. Of them, smeU1VWU2X operon is unique for its possession of two additional genes, smeU1 and smeU2, which encode proteins of short-chain dehydrogenase/reductase (SDR) family. Overexpression of the SmeVWX pump is known to contribute to the acquired resistance to chloramphenicol, quinolone, and tetracycline; however, SmeU1 and SmeU2 are little involved in this phenotype. In this article, we further linked smeU1VWU2X operon to oxidative stress alleviation and sulfamethoxazole/trimethoprim (SXT)-resistant mutants occurrence. SmeU1VWU2X operon was inducibly expressed upon the challenge of menadione (MD), plumbagin (PL), and hydrogen peroxide (H2O2), verified by chromosomal smeU1VWU2X-xylE transcriptional fusion construct and quantitative real-time PCR (qRT-PCR). The MD-mediated smeU1VWU2X upexpression was totally dependent on SoxR, partially relied on SmeRv, but was less relevant to OxyR. SmeRv, but not SoxR and OxyR, played a regulatory role in the H2O2-mediated smeU1VWU2X upexpression. The significance of smeU1VWU2X upexpression was investigated in respect of oxidative stress alleviation and SXT-resistant mutants occurrence. Overexpression of smeU1VWU2X operon contributed to the alleviation of MD-mediated oxidative stress. Of the encoded proteins, SmeVWX pump and SmeU2, rather than SmeU1, participated in the MD tolerance. Furthermore, we also demonstrated that MD-mediated expression of smeU1VWU2X operon decreased SXT resistance frequency when S. maltophilia was grown in the reactive oxygen species (ROS)-rich environment.
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