Effects of nitrite and nitrate on the growth and acidogenicity of Streptococcus mutans

It is hypothesised that exogenous nitrite acidified by metabolic products of acidogenic bacteria in the mouth will be converted to products which inhibit growth of the bacteria in question which contribute to dental caries. Objectives. The aims of this study were (1) to test the activity of both sodium nitrate and sodium nitrite at differing concentrations on the ability of Streptococcus mutans to lower the pH of its surroundings and hence (2) to determine whether either nitrate or nitrite might be bactericidal or bacteriostatic against S. mutans. Methods. S. mutans NCTC 10449T was cultured in a liquid medium to which either sodium nitrate or sodium nitrite was added to a final concentration of 0.0, 0.2, 2.0, 20 or 200 mM, of which the first acted as a test substance negative control. After 24 h, the cultures were streaked onto agar to test for growth and the remaining culture used for pH measurement. The Mann–Whitney U-Test was used for statistical comparison of pH values. Results. Nitrite at concentrations of 20 and 200 mM had a highly significant inhibitory effect (p<0.001) on the ability of S. mutans NCTC 10449T to lower pH. Moreover, bacteria that had been subjected to these levels of nitrite were unable to recover on solid medium. Nitrate had no such effect on either the growth of the bacteria or on their ability to lower pH. Conclusions. It is concluded that nitrite, at final concentrations of either 20 or 200 mM, is both bactericidal and anti-acidogenic with respect to S. mutans, while lower concentrations of nitrite and all concentrations of nitrate are ineffective. Nitrite might be worthy of consideration as a mouth-rinse constituent.