One preventive effect of topical fluoride application is derived from the fact that fluoride can inhibit bacterial acid production. Furthermore, divalent cations such as Ca2+ and Mg2+ increase the binding of fluoride to bacterial cells. These findings suggest that exposure of oral bacteria to fluoride in the presence of divalent cations increases fluoride binding to bacterial cells and subsequently enhances fluoride-induced inhibition of bacterial acid production. This study investigated the effects of fluoride exposure (0-20,000 ppm F) in the presence of Ca2+ or Mg2+ prior to glucose challenge on pH fall ability by bacterial sugar fermentation, as well as fluoride binding to bacterial cells by exposure to fluoride, and fluoride release from bacterial cells during bacterial sugar fermentation, using caries-related bacteria, Streptococcus mutans and Streptococcus sanguinis. The pH fall by both streptococci was inhibited by exposure to over 250 ppm F in the presence of Ca2+ (p < 0.01), whereas in the presence of Mg 2+, the pH fall by S. mutans and S. sanguinis was inhibited after exposure to over 250 and 950 ppm F, respectively (p < 0.05). The amounts of fluoride binding to and released from streptococcal cells increased with the concentration of fluoride the cells were exposed to in the presence of Mg 2+, but were high enough even after 250 ppm F exposure in the presence of Ca2+. The enhanced inhibition of acid production in the presence of divalent cations is probably due to the improved efficiency of fluoride binding to bacterial cells being improved via these divalent cations.
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