Oxidative Stressors Modify the Response of Streptococcus mutans to Its Competence Signal Peptides

The dental caries pathogen is continually exposed to several types of stress in the oral biofilm environment. Oxidative stress generated by reactive oxygen species has a major impact on the establishment, persistence, and virulence of Here, we combined fluorescent reporter-promoter fusions with single-cell imaging to study the effects of reactive oxygen species on activation of genetic competence in Exposure to paraquat, which generates superoxide anion, produced a qualitatively different effect on activation of expression of the gene for the master competence regulator, ComX, than did treatment with hydrogen peroxide (H O ), which can yield hydroxyl radical. Paraquat suppressed peptide-mediated induction of in a progressive and cumulative fashion, whereas the response to H O displayed a strong threshold behavior. Low concentrations of H O had little effect on induction of or the bacteriocin gene , but expression of these genes declined sharply if extracellular H O exceeded a threshold concentration. These effects were not due to decreased reporter gene fluorescence. Two different threshold concentrations were observed in the response to H O , depending on the gene promoter that was analyzed and the pathway by which the competence regulon was stimulated. The results show that paraquat and H O affect the competence signaling pathway differently, and that some portions of the competence signaling pathway are more sensitive to oxidative stress than others. inhabits the oral biofilm, where it plays an important role in the development of dental caries. Environmental stresses such as oxidative stress influence the growth of and its important virulence-associated behaviors, such as genetic competence. competence development is a complex behavior that involves two different signaling peptides and can exhibit cell-to-cell heterogeneity. Although oxidative stress is known to influence competence, it is not understood how oxidative stress interacts with the peptide signaling or affects heterogeneity. In this study, we used fluorescent reporters to probe the effect of reactive oxygen species on competence signaling at the single-cell level. Our data show that different reactive oxygen species have different effects on competence, and that some portions of the signaling pathway are more acutely sensitive to oxidative stress than others.