The effect of fluoride on the stability of oral bacterial communities in vitro.

An in vitro model system has been used to study the combined influences of fermentable carbohydrate, pH, and fluoride on the stability of complex oral microbial communities. The pH generated from carbohydrate pulses rather than the availability of substrate per se was responsible for the enrichment of the cariogenic species S. mutans and L. casei. The addition of sub-MIC levels of sodium fluoride (1 mmol/L; 19 ppm) reduced both the rate of acid production and the fall in terminal pH from glucose pulses, thereby enabling pH-sensitive bacteria, including many Gram-negative species, to persist. Furthermore, the combination of even a moderately-low environmental pH (ca. pH 5.0) with a low level (1 mmol/L) of fluoride was able to prevent S. mutans from out-competing other species and resulted in its proportions within the bacterial community remaining low. By this mechanism, fluoride could make a significant contribution to preventing dental caries.