Effect of Sodium Nitrite and Ascorbic Acid on Streptococcus mutans

Objectives: Nitrite at acid pH has been shown to be anti-bacterial particularly against the highly cariogenic species Streptococcus mutans. There are some concerns about ill health effects to humans from exposure to nitrite and it has been suggested that ascorbic acid (vitamin C) could counteract these ill effects. The aim of this study was to test the effect of ascorbic acid used with, and without, sodium nitrite on the growth and acid production of S. mutans, NCTC 10449^T and NCTC 10832. Methods: Strains were grown in Fastidious Anaerobe Broth before exposure to 200 mM nitrite and various concentrations of ascorbic acid (0.0, 0.1, 0.12, 0.175, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5%). End pH was measured after 24 h exposure and cultures were serially diluted for viable counts. Statistical significance of differences in end pH values, each from ten experiments, was determined using the Mann-Whitney U-Test. Results: At all concentrations of ascorbic acid tested without nitrite the pH was reduced to between pH 4.32 and 4.39 as with ascorbic acid free controls. At all concentrations of ascorbic acid plus 200 mM nitrite S. mutans failed to lower the pH (6.53-6.69), compared with controls lacking ascorbic acid and/or nitrite (p³ 0.001). In fact with ascorbic acid and 200 mM nitrite end pH was an average 6.65 compared with 200 mM nitrite alone which gave an average end pH of 6.53. S. mutans was viable (between 1.3 x 10⁹ and 5.0 x 10⁹ cfu ml^-1) after exposure to ascorbic acid at all concentrations without nitrite. Conclusions: Ascorbic acid does not interfere in the action of nitrite against the growth and acid production of S. mutans and possibly even enhances the effect against acid production. Therefore ascorbic acid could be used in conjunction with nitrite to help counteract potential ill effects for humans.