Effects of fluoride on the microbial ecology of dental plaque

In order to survive in a community such as dental plaque, bacteria must be able to resist changes in the environment. These changes may be the result of differences in the host or the activities of the bacteria within the community. Bacteria which cannot resist these environmental pressures may cease metabolism but survive, or be eliminated from the community. Fluoride is only one of numerous environmental pressures which impact on oral bacterial communities, yet its beneficial effect on caries is such that it could be proposed that in addition to its effects on enamel and remineralization, fluoride would affect the bacteria. It has been shown on many occasions that fluoride can affect bacterial carbohydrate fermentation; however, are there other effects of fluoride on the bacteriology of dental plaque? Although very high levels of fluoride (0.16-0.3 mol/L) will kill bacteria, there is little evidence that fluoride causes dramatic changes in the number of species found in plaque or their relative concentrations. The survival of the populations in the plaque community in the presence of fluoride could result from: (1) the fact that all the organisms are unaffected, (2) selection of mutant strains resistant to fluoride, or (3) adaptation of strains to fluoride resistance. Examples of these phenomena can be found among members of the oral flora. Some species are unaffected (Veillonella), but Streptococcus can be mutated to fluoride resistance and also adapt phenotypically to increasing levels of fluoride in the environment. Which of these mechanisms operates in vivo, and does resistance, mutation, or phenotypic adaptation of these bacteria reduce the effectiveness of fluoride, in controlling caries?(ABSTRACT TRUNCATED AT 250 WORDS)