Divalent cations enhance short-time fluoride exposure-induced inhibition on streptococcal acidogenicity

Objective: It has been demonstrated that divalent cations such as Ca²⁺ and Mg²⁺ increase the fluoride-binding of bacteria, but the effect of a short-time exposure of fluoride on oral bacteria in the presence of divalent cations is unclear. Thus, this study aimed to evaluate the effects of divalent metal ions on the short-time fluoride exposure-induced inhibition of the acid production of streptococci. Methods: Streptococcus mutans NCTC 10449 and Streptococcus sanguinis ATCC 10556 cells were exposed to KF (F=0 (control), 450 and 950 ppm) at pH 7.0 for 10 min and washed. The cells were incubated with 28 mM glucose and the pH fall was monitored for 40 min with a fluoride electrode. 5 mM MgCl₂-, 5 mM CaCl₂-containing or divalent cation-free phosphate buffer (pH 7.0) were used throughout the experiments. Results: The minimum pH values of S. mutans with and without fluoride exposure (F=450 and 950 ppm) in the absence of divalent cation were 4.0±0.1 and 4.2±0.1, respectively. In contrast, the minimum pH values of S. mutans with the fluoride exposure at 450 and 950 ppm in the presence of Mg²⁺were increased (P<0.001) to 4.8±0.2 and 5.0±0.1, respectively, and those in the presence of Ca²⁺ were increased (P<0.001) to 5.1±0.1 and 5.1±0.1, respectively. Similar results were obtained for S. sanguinis, although the minimum pH values were relatively higher than those of S. mutans. Conclusion: This study revealed that the short-time fluoride exposure inhibits the acid production of streptococci in the presence of divalent cations, probably because of fluoride can bind to bacterial cells efficiency via divalent cations. Therefore, it is expected that a short-time topical fluoride application together with divalent cations can reduce the risk of caries in terms of inhibitory effect on acid production by plaque bacteria in vivo.