Relationship between Fluoride Concentration and Activity against Virulence Factors and Viability of a Cariogenic Biofilm: in vitro Study

Abstract

Despite widespread use of various concentrations of fluoride for the prevention of dental caries, the relationship between fluoride concentration and activity against cariogenic biofilms has not been much studied. Herein we investigated the relationship between fluoride concentration and activity against virulence factors and viability of Streptococcus mutans biofilms. S. mutans biofilms were formed on saliva-coated hydroxyapatite discs. The 70-hour-old biofilms were exposed to 0, 1, 3, 10, 30, 100, 300, 1,000 or 2,000 ppm F^-. The changes of virulence factors and viability of the biofilms were analyzed using biochemical methods and laser scanning confocal fluorescence microscopy. At 1-2,000 ppm F^-, the activity of fluoride against acid production, acid tolerance, and extracellular polysaccharide formation of S. mutans biofilms accurately followed a sigmoidal pattern of concentration dependence (R² = 0.94-0.99), with EC₅₀ values ranging from 3.07 to 24.7 ppm F^-. Generally, the activity of fluoride against the virulence factors was concentration-dependently augmented in 10-100 ppm F^- and did not increase further at concentrations higher than 100 ppm F^-. However, fluoride did not alter glucosyltransferase activity and viability of S. mutans biofilm cells in all concentrations tested. These results can provide a basis for the selection of appropriate fluoride concentrations that reduce the physiological ability of cariogenic biofilms.