Effect of pH on Fluoride Penetration into Natural Plaque Biofilms

Introduction: Extensive uptake and penetration of fluoride into plaque is essential for its effectiveness in encouraging remineralisation and inhibiting demineralisation. Recent work has illustrated that, during short periods associated with tooth brushing, fluoride penetration is limited to the outer third or so of the plaque layer. The reasons are not clear but may be due to charge interactions with biomass components. Low pH was therefore used to examine the penetration of unionised i.e. protonated fluoride through plaque. Objectives: To examine the effect of low pH on the penetration of fluoride into natural plaque biofilms. Methods: Natural plaque biofilms were produced on natural enamel surfaces in vivo using the Leeds plaque generation device. After 7 or 21 days, devices were recovered from the mouth and exposed for 30 seconds to 1000ppm fluoride at either pH 3 or pH 7. Plaque biofilms were subsequently lyophilised, embedded in methacrylate and sectioned at 5 µm intervals from the plaque/saliva interface to the enamel surface. Fluoride concentrations were determined in the sections and the concentration profiles of fluoride through the plaque calculated. Results: Control and experimental plaque biofilms showed a limited penetration of fluoride to the outer regions of the plaque as described previously. However, plaque exposed to fluoride at pH 3 showed significantly less fluoride throughout the biofilm. This was slightly more obvious in the 21 day plaques. Conclusions: At low pH fluoride may either diffuse less readily into plaque due to binding with biomass components and/or be rapidly leached from the plaque biofilm. This may also be due an effect of low pH on the biofilm itself. The implications for acidification of plaque due to dietary components may be profound.