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Background   Chromobacterium violaceum  is an environmental opportunistic pathogen that causes rare but deadly infections in humans. The transcriptional regulators that  C. violaceum  uses to sense and respond to environmental cues remain largely unknown.    Results  Here, we described a novel transcriptional regulator in  C. violaceum  belonging to the MarR family that we named OsbR (oxidative stress response and biofilm formation regulator). Transcriptome profiling by DNA microarray using strains with deletion or overexpression of  osbR  showed that OsbR exerts a global regulatory role in  C. violaceum , regulating genes involved in oxidative stress response, nitrate reduction, biofilm formation, and several metabolic pathways. EMSA assays showed that OsbR binds to the promoter regions of several OsbR-regulated genes, and the in vitro DNA binding activity was inhibited by oxidants. We demonstrated that the overexpression of  osbR  caused activation of  ohrA  even in the presence of the repressor OhrR, which resulted in improved growth under organic hydroperoxide treatment, as seem by growth curve assays. We showed that the proper regulation of the  nar  genes by OsbR ensures optimal growth of  C. violaceum  under anaerobic conditions by tuning the reduction of nitrate to nitrite. Finally, the  osbR  overexpressing strain showed a reduction in biofilm formation, and this phenotype correlated with the OsbR-mediated repression of two gene clusters encoding putative adhesins.    Conclusions  Together, our data indicated that OsbR is a MarR-type regulator that controls the expression of a large number of genes in  C. violaceum , thereby contributing to oxidative stress defense ( ohrA / ohrR ), anaerobic respiration ( narK1K2  and  narGHJI ), and biofilm formation (putative RTX adhesins).    Supplementary Information  The online version contains supplementary material available at 10.1186/s12866-021-02369-x.

The MarR family regulator OsbR controls oxidative stress response, anaerobic nitrate respiration, and biofilm formation in Chromobacterium violaceum