Effects of Fluoride on the Microbial Ecology of Dental Plaque

Abstract

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? It must be recognized that the evidence for mutation or phenotypic adaptation of oral bacteria to fluoride resistance in vivo is not conclusive. In addition, if organisms can adapt phenotypically, the selection of resistant mutants may not occur. Moreover, strains adapted in vitro or in experimental animals can show a reduced rate of carbohydrate metabolism. This suggests that ‘adapted strains’ may be of reduced cariogenic potential. To date, studies indicate that fluoride at prophylactic levels in the environment does not influence the composition of dental plaque. Its action is on the metabolic capacity of cells, causing a reduction in the rate of acid production. In vivo adaptation or mutation has not been shown conclusively, and bacteria naturally resistant to fluoride are probably under the control of other environmental pressures. Fluoride acts, therefore, by stabilizing the ecosystem, including the habitat (tooth tissue) and carbohydrate metabolism of the community. The increase in plaque pH brought about by the latter effect not only protects enamel but also reduces any advantage given to aciduric bacteria by carbohydrate metabolism.