The Effect of Environmental Factors on the Nitrate and Nitrite Metabolism of Oral <i>Actinomyces</i> and <i>Schaalia</i> Species

Actinomyces naeslundii and Schaalia odontolytica belong to the most predominant nitrite-producing bacteria in the oral microbiome. Nitrite has antibacterial and vasodilatory effects that may contribute to maintaining oral and systemic health. We have previously elucidated the metabolic characteristics of the nitrite-producing activity of oral Veillonella species and the effects of oral environmental factors. However, this is still unknown for Actinomyces and Schaalia species. Furthermore, these bacteria are thought to degrade nitrite. Therefore, this study aimed to comprehensively elucidate the effects of environmental factors (pH, oxygen concentration, glucose, lactate, and the presence of nitrate/nitrite during growth) on nitrate and nitrite metabolism of these bacterial species using the type strains. Nitrite was quantified by Griess reagent, and final metabolites were analyzed by high-performance liquid chromatography (HPLC). The nitrite-producing activity of A. naeslundii and S. odontolytica was affected variously by environmental factors. Especially in A. naeslundii, under anaerobic conditions, the activity increased in a concentration-dependent manner with the addition of glucose or lactate and was higher at lower pH when lactate was added. The nitrite-degrading activity of both bacteria was lower than the nitrite-producing activity and was less affected by environmental factors. Metabolites from glucose by A. naeslundii were different with and without nitrate, suggesting that nitrate altered metabolic pathways. The growth was inhibited under anaerobic conditions but promoted under aerobic conditions. These results indicate that the nitrite-producing capacity of the oral microflora must take into account not only the composition and abundance of bacteria but also the variation in metabolic activity due to various environmental factors.