Acidogenesis in relation to fluoride resistance of Streptococcus mutans

The velocity of acid production (Vap) of Streptococcus mutans C180-2 and of 2 fluoride-resistant mutant strains S. mutans C180-2FR and S. mutans C180-2MFR was examined in vitro at pH values between 7.0 and 4.5. The Vap of the fluoride-resistant mutants was lower than the Vap of the parent strain at pH greater than or equal to 6.0. At pH 5.5 and 5.0, the Vap of the mutant strains was higher than the Vap of the parent strain, whereas at pH 4.5 no significant differences were observed between the Vap of the 3 strains. The fluoride sensitivity of all 3 strains was amplified by a low pH environment. The fluoride concentration necessary to inhibit the acid production of the parent strain completely was 27 mM at pH 7 and 0.1 mM at pH 4.5. For the 2 mutants, the corresponding concentrations were 65 mM at pH 7 and 0.5-0.7 mM at pH 4.5. The results suggest that, if S. mutans acquires fluoride resistance in vivo, the rate of acid production in dental plaque may be decreased at pH greater than or equal to 6, but increased at lower pH levels. Low concentrations of fluoride inhibit acid production less effectively.