OhrRA functions as a redox-responsive system controlling toxinogenesis in Bacillus cereus

Bacillus cereus OhrR is a member of the subgroup of the MarR (multiple antibiotic resistance) family of transcriptional regulators that use a cysteine-based redox sensing mechanism. OhrA is a thiol-dependent, peroxidase-like protein. The dual OhrRA system triggers B. cereus adaptation in response to redox changes, such as those encountered in the environment of the gastrointestinal tract. Here, we investigated the role of OhrRA in toxinogenesis. Comparative shotgun analysis of exoproteomes from ∆ohrA, ∆ohrR and wild-type cells revealed significant changes in the abundance levels of toxin-related proteins depending on the extracellular redox potential. We complemented these data by measuring the DNA binding activity of reduced and oxidized recombinant OhrR on toxin and putative toxin promoter regions. Furthermore, transcriptomic data and OhrRA-dependent, antiproliferative activity of the B. cereus exoproteome on Caco-2 human epithelial cells were recorded. The results indicate that OhrR controlled toxin gene expression directly or indirectly in a redox- and toxin-dependent manner, and may function as a repressor or an activator. Moreover, we found that OhrR restricts toxin-dependent antiproliferative activity of the B. cereus exoproteome whatever the growth conditions, while the restrictive impact of OhrA occurs only under low ORP anoxic conditions.

B. cereus is a notorious foodborne pathogen which causes gastroenteritis. Fatal and severe cases have been reported. The pathogenicity of B. cereus is intimately associated with the production of epithelial cell-destructive toxins in the small intestine. The small intestine poses several challenges for a pathogen because it is sliced into various niches with different oxygen concentrations and different redox potentials. We recently showed that the organic hydroperoxide resistance OhrRA system was crucial to the successful adaptation of B. cereus to extreme redox environments such as those encountered in the lumen (high reducing anoxic environment) and on the intestinal epithelium (transient oxic environment). Here we provide evidence that this bacterial system is a major virulence determinant in B. cereus in that it coordinates toxinogenesis in a redox dependent manner. Specifically, our comparative exoproteomic analyses reveal that OhrR strongly restricts B. cereus toxinogenesis under high reducing anoxic conditions while OhrA boosts toxinogenesis. Based on exoproteomic analyses, we further examined the role of OhrR and found that it functions as a redox-dependent transcriptional regulator of toxin and putative toxin genes. These findings provide novel insights into the weapons used by B. cereus to control its toxinogenic potential and, as a result its toxicity against human epithelial cells.