Sugar, fluoride, pH and microbial homeostasis in dental plaque

Factors that may contribute to the maintenance or breakdown of the oral microflora have been studied in the laboratory using a mixed culture chemostat system. Carbohydrate type had relatively little influence of the proportions of individual species at neutral pH. In contrast, when the pH was allowed to fall following carbohydrate metabolism, the stability of the microflora was markedly perturbed. The proportions of Streptococcus mutans, Lactobacillus casei and Veillonella dispar increased and they became the predominant species, while levels of other Gram-negative organisms and S. gordonii declined. Low levels (1 mmol/l) of sodium fluoride (NaF) had little effect on the microflora at neutral pH. However, when the pH fell following carbohydrate metabolism, 1 mmol/l of NaF slowed acid production and, in so doing, reduced the inhibition of acid-sensitive species, and suppressed the selection of S. mutans under otherwise favourable conditions. These data (and others) suggest that the mode of action of fluoride in human beings might include a subtle but clinically significant antimicrobial effect. These findings have also led to the proposal of a modified hypothesis (the "ecological plaque hypothesis") to explain the role of the resident oral microflora in dental disease. The hypothesis also has implications for treatment and prevention strategies.