| Streptococcus mutans, an oral bacterium found on human tooth surfaces, is a primary causative agent of the disease dental caries. Dental caries is the result of the demineralization of the tooth surface, partially due to the production of acid by oral bacteria through carbohydrate metabolism. Rapid sugar metabolism in S. mutans depends on the availability of reduced nicotinamide dinucleotides (NADH). A key enzyme for the regeneration of NADH is the flavin-containing NADH oxidase (Nox). This enzyme oxidizes NADH to NAD+, while reducing diatomic oxygen to H2O. In this study, we characterized the role of the NADH oxidase in the oxidative and acid stress responses of S. mutans, determined specific regulatory controls of NADH oxidase and described the global regulatory effects from the loss of nox. The nox mutant strain exhibited reduced ability to metabolize environmental oxygen present in chemostat-grown S. mutans cultures resulting in activation of the oxygen and acid-mediated stress responses, as demonstrated by elevated activity of superoxide dismutase and glutathione oxidoreductase, elevated transcription of DNA repair genes and altered membrane fatty acid composition, independent of external pH. An Spx recognition site was identified within the first 120 bp upstream of the translational start site of the nox-coding region. Measurements of transcription rates from the nox promoter showed that SpxA activates nox and that SpxB slightly inhibits nox, indicating that nox is part of the Spx global regulon. The global effects of the nox mutation and the impacts of oxygen were characterized using cDNA microarrays. Transcriptional patterns were similar between the nox mutant and parent strain, UA159, when exposed to elevated oxygen concentrations, with some exceptions. Notably, in the nox mutant strain, the global regulator Rex, responsible for regulation of oxygen homeostasis, sugar metabolism, and biofilm formation, was differentially regulated, suggesting that Nox and Rex participate in a redox-sensing and signaling pathway. In conclusion, nox is a major consumer of environmental oxygen in S. mutans and the loss of NADH oxidase, or an increase in environmental oxygen, leads to global transcriptional changes, allowing the organism to respond rapidly to stress at the expense of overall growth rate. |
