Sodium Nitrite, Sorbitol and the Acid Production of Streptococcus mutans

Objectives: Nitrite at acid pH has been shown to kill Streptococcus mutans. Previously S. mutans has been grown in broth supplemented with glucose as a fermentable substrate. Aims were a) to confirm the effect of using sorbitol on the ability of S. mutans to lower the pH and b) to test the effect of nitrite on S. mutans growing in sorbitol-supplemented broth. Methods: S. mutans NCTC 10449^T and NCTC 10832 were grown up in Fastidious Anaerobe Broth containing a range of sorbitol concentrations from 0.04% to 1.0% w/v for 24 h before end pH was measured. Nitrite was tested at final concentrations of 0, 0.2, 2.0, 20 and 200 mM. Again end pH was measured after 24 h exposure and cultures were serially diluted for viable counts. Experiments were repeated ten times. Results: At all concentrations of sorbitol used, S. mutans was able to lower the pH of the broth to below pH 5.5 and at concentrations of 1.0%, 0.5% and 0.2% the final pH was below 5.0. These results are similar to those from controls using glucose as the fermentable substrate. At all concentrations of sorbitol tested, acid production was highly significantly reduced (compared to controls with no nitrite present) by addition of 20 mM (M-W U-Test, p³ 0.001) and 200 mM (M-W U-Test, p³ 0.001) nitrite. At 0.5% and 1.0% sorbitol 2.0 mM nitrite also significantly reduced acid production (M-W U-Test, p=0.01). S. mutans was viable after exposure to nitrite but cfu ml^-1 were significantly less than for S. mutans grown in broth plus sorbitol alone. Conclusions: 20 and 200 mM nitrite significantly decrease acid production from sorbitol by S. mutans. Furthermore 200 mM nitrite produces a 95% reduction in the viable cell count of S. mutans.