Effect of NaF on biofilm microbiota and enamel demineralization

Objective: Fluoride is widely used as an anti-caries agent, e.g. in toothpastes and mouthrinses. However, the nature of the anti-caries action is not entirely clear. The aim of this study was to examine with the Zurich in vitro biofilm model the effect of fluoride on the microbiota and on demineralization. Methods: Biofilms consisting of six oral species, namely Actinomyces naeslundii, Candida albicans, Fusobacterium nucleatum, Streptococcus oralis, Veillonella dispar and Streptococcus sobrinus, were grown anaerobically on sintered hydroxyapatite or bovine enamel disks for 64.5 h at 37°C in 70% saliva and 30% medium (mFUM). Biofilms were exposed to 200, 400, and 1400 ppm of fluoride at regular intervals. Chlorhexidine (0.1%) and water were used as controls. The additional buffering of mFUM with Sørensens PBS was omitted for demineralization experiments. Biofilms were harvested after 64.5 h and assessed by culture for total CFU, total streptococcal CFU, and CFU of all individual taxa. Demineralization of enamel disks was measured by quantitative light-induced fluorescence (QLF). Results: At 1400 and 400 ppm fluoride neither affected the total number of CFU nor the CFU of the individual species. At 200 ppm it resulted in a weak reduction of F. nucleatum and V. dispar CFUs reaching borderline significance. No mineral loss of enamel disks was observed at 1400 and 400 ppm of fluoride, although the pH in the surrounding medium had dropped from 7.2 to 5.5 and 5.0, respectively. At 200 ppm of fluoride the pH had fallen to 4.5, accompanied by a small mineral loss (ΔF=-6.6%). In comparison, the water control showed a ΔF of -32%. Conclusions: As fluoride completely prevented demineralization without affecting biofilm composition and growth or acid production, the anti-caries effect of fluoride must rely on a shift of the de/remineralization balance resulting in increased enamel resistance.