The effect of fluorhydroxyapatite-derived fluoride on acid production by streptococci

The effect of fluoride derived from fluorhydroxyapatite (FHAp) minerals on bacterial glycolysis under aerobic and strictly anaerobic conditions was studied to validate the claims that this mineral could be used as a reservoir of fluoride in plaque. To isolate the direct effect of fluoride on bacterial glycolysis from that of an indirect pH-buffering effect of hydroxyl or phosphate ions which are also dissolved from the mineral, we equalized the pH-fall time course of reactions by manually adding KOH or HCl. This ensured that pH effects on glycolysis were minimized. Under controlled pH-fall and strictly anaerobic conditions, fluoride derived from the dissolution of FHAp containing more than 30,100 ppm fluoride (i.e., when the substitution of OH by F in the mineral was greater than 80%) had a direct inhibitory effect on lactic acid production in Streptococcus mutans. Under free pH-fall and strictly anaerobic conditions, increasing amounts of fluoride in FHAp (starting as low as 2000 ppm fluoride), appeared to have a pronounced indirect inhibitory effect on lactic acid production. This was probably mediated through a reducing pH buffer effect of the mineral. Even in the presence of high-fluoride FHAp, only 0.01 to 0.025 mmol/L fluoride was found in the reaction mixtures, a probable result of non-stoichiometric dissolution of FHAp. In spite of such low levels of fluoride, marked inhibitory effects on bacterial glycolysis were demonstrated. The results of this study suggest that high-fluoride FHAp may serve as a reservoir of fluoride for the inhibition of anaerobic acid production by S. mutans.