The antimicrobial action of fluoride and its role in caries inhibition

Despite a considerable amount of literature on the effects of fluoride in dental plaque, several urgent questions remain unanswered, such as: Does the inhibiting effect of fluoride on dental plaque metabolism contribute to caries prevention? Does adaptation of plaque to fluoride affect its cariogenicity? Single applications of fluoride directly to dental plaque reduced acid production. Also, fluoride dissolving from topically treated enamel reduced the acid production in covering layers of oral bacteria in vitro. The effects of both treatments were only of short duration and may not be relevant to caries prevention in vivo. In contrast, daily applications of fluoride resulted in a reduction of the acidogenicity of dental plaque even 8-12 h after the treatment. Such a reduction is likely to contribute to caries prevention. But it has to be realized that when plaque reaches saturation with respect to fluoridated calcium (phosphate) precipitates, enamel becomes insoluble and any antimicrobial effect becomes irrelevant. Still lacking are data on the antimicrobial effects of fluoride regimens normally used in home care, in weekly rinsing programs in schools, or treatments applied professionally every six months. Adaptation of Streptococcus mutans to fluoride has been suggested to reduce the cariogenic potential of the cells. In vitro-induced fluoride-resistant strains were less cariogenic in rats, and the velocity of acid production in vitro was reduced at constant pH greater than 5.5. Despite the ability of oral bacteria to adapt to fluoride, evidence of adaptation in dental plaque of normal subjects resulting in a reduced cariogenic potential has not yet been demonstrated.