Biological Effects of Fluoride on Dental Biofilms
Bacteria adhering to a surface form complex matrix-embedded communities called biofilms. These communities are difficult to dislodge and play an important role in the development of infection. The bacteria that inhabit your mouth can colonize your teeth to form biofilms called plaque. Because some of these bacteria metabolize sugar to release lactic acid as a byproduct, they create an acidic environment in the biofilm that leaches the minerals from your teeth. The evidence suggests fluoride can help inhibit biofilm growth.
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Direct Inhibition
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Fluoride ions can directly inhibit certain bacterial enzymes in some of the bacteria in your mouth or form metal complexes that inhibit enzyme activity. Fluoride ions, however, do not readily penetrate cell membranes, so the extent of this effect is modest.
Acidification
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Fluoride ions are a conjugate base of a weak acidic, hydrofluoric acid or hydrogen fluoride. The environment in a biofilm is often very acidic because plaque bacteria release acids as a byproduct of their metabolism. At these low pH levels, the equilibrium between fluoride ions and hydrogen fluoride in solution shifts toward hydrogen fluoride, and fluoride ions will accept hydrogen ions to become hydrogen fluoride, or HF. HF is better able to penetrate bacterial cell membranes than the fluoride ion because it has no net charge. Once it does so, it will dissociate in the cell cytoplasm to release hydrogen ions, thus decreasing the pH. This series of events helps to disrupt acid production by the bacteria and increases the stress on the bacteria, slowing or inhibiting bacterial growth.
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Penetration
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The most important effect of fluoride is to promote the remineralization of your teeth and inhibit demineralization. Both the effectiveness of fluoride in strengthening your teeth and inhibiting bacterial growth is constrained by the limited extent to which fluoride penetrates the biofilm. According to a 2005 study in the Journal of Dental Research, during the short length of time you spend brushing your teeth, fluoride doesn't have an opportunity to completely penetrate the biofilm, so while outer layers of the biofilm receive more fluoride, those closest to the teeth receive less. Spending more time brushing may help increase fluoride penetration.
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References
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