Acidic pH Enhances Fluoride Binding on Streptococcus mutans.

Method: S. mutans clinical isolates were grown in BHI medium at 37^oC for 18 hours, washed twice with sterile phosphate buffer, followed by sonication in water bath for 10 min. The bacteria at a density of 1 x 10⁹ were washed either with EDTA or CaCl₂ both at 1 mM to remove or add calcium, respectively. After centrifugation at 5000 rpm for 5 min, the pellets were incubated with 12 mM NaF at pH 4.5 or pH 7 for 1 min in order to simulate the time of brushing/mouthwashing. They were centrifuged and the supernatant was collected for F quantification. Pure water was added to the pellets and the lysates were sonicated (10 pulses) and removed for further F measurement. Lysates and supernatants were analyzed for F with the electrode, after HMDS-facilitated diffusion. Data were tested by 2-way ANOVA followed by Bonferroni’s test (p<0.05).

Result: Both condition (lysate or supernatant) and pH presented significant interaction. Significantly higher F amounts were detected in the lysates of bacteria incubated with NaF solution at pH 4.5 compared to the supernatant, in presence of CaCl₂.  However, at pH 7.0 in presence of calcium, the supernatant presented significant higher F amounts compared to the respective lysate. In absence of calcium, we detected higher concentration of F (p<0.01) in the supernatant compared to lysate in both pH evaluated and no differences in F levels were found between lysates at pH 4.5 and pH 7.0.

Conclusion: The results show that F at low pH binds more efficiently to S. mutans than at neutral pH in a calcium-dependent manner.