Nitrite reductase in Streptoccocus mutans plays a critical role in the survival of this pathogen in oral cavity.

Background/aims: The mechanisms of nitric oxide (NO) production by bacteria in the oral cavity are still not clearly defined but salivary streptococci have been reported to generate NO. The aim of this study was to clarify the mechanism of nitrite metabolism and generation of NO by Streptococcus mutans, a major pathogen of dental caries. Methods: We searched the genomic database of oral pathogens for nitrite reductase and used a polymerase chain reaction (PCR) to clone the nirJ gene from S. mutans GS5. His-tagged recombinant NirJ protein was expressed in Escherichia coli BL21 and characterized. We constructed a nirJ gene-disrupted mutant strain of S. mutans (ΔnirJ) to analyze the physiological significance of nirJ. Results: S. mutans generates NO from nitrite, probably as a result of the possession of nitrite reductase. We cloned the nirJ gene from S. mutans GS5 by PCR. The recombinant NirJ protein catalyzed the reduction of nitrite with a K_m value of 3.37 μm and a specific activity of 2.5 μmol/min/mg of protein at 37°C. Biochemical analysis revealed that the nitrite-reducing activity of the mutant (ΔnirJ) strain was significantly lower than that of the wild-type strain. The growth of the mutant strain, but not of the wild-type strain, was strongly suppressed by the presence of physiological levels of nitrite (∼0.2 mm) in saliva. Conclusion: These observations suggest that the elimination of nitrite and/or the generation of NO are important for the survival of S. mutans in the oral cavity. [ABSTRACT FROM AUTHOR]