Effect of fluoride and xylitol on the growth and acid production by Streptococcus mutans

The main effect of fluoride (F) on dental caries control is physicochemical, but after a mouth rinse (0.05%NaF) an antibacterial concentration (~50ppmF) is reached in dental plaque. Xylitol (X) is an effective anti-caries agent and has shown inhibitory effect on oral bacteria. However, the effect of XF combination is not clearly established. Objectives: To evaluate the minimum concentration of xylitol into fluoride mouth rinses to exert inhibitory effect on the growth and acid production by mutans streptococci. Methods: Streptococcus mutans IB1600 was grown in brain-heart infusion broth (BHI) with various xylitol concentrations (0.05–0.8M) and containing or not 56ppmF (NaF). The effect of these substances on the bacterial growth was evaluated under two different conditions: (1) S.mutans IB1600 was inoculated into BHI containing X, F, or XF; (2) S.mutans IB1600 was inoculated into BHI but X, F, or XF was added 3 h after inoculation (middle of log phase). The cultures were incubated at 37^oC, 10%CO₂ and the growth was assessed at 650 nm, every two hours for 24h. In sucrose fermentation test, standardized bacterial cell suspensions (1-2x10⁸ cfu/mL) were used and the pH was measured every 15 min for 1h. The difference from the initial to the end pH was calculated. Results: In growth study (1), the inhibitory effect of 0.1M XF was not different from 0.8M X(p>0.05). In growth study (2), a similar inhibitory effect was found in a two-time higher xylitol-fluoride concentration (0.2M XF). There is no correlation between X concentrations and pH decrease (p>0.05), but it was significant when X was combined with F. Conclusion: The data suggest that the concentration of xylitol in combination with fluoride promoted the same inhibitory effect on mutans growth than xylitol alone at a concentration 4-8-times higher. In addition, the combination of xylitol and fluoride inhibited the pH drop when compared to xylitol alone.(Supported by FAPESP 00/00750-0).