Analysis of pH-driven disruption of oral microbial communities in vitro

Previously, a mixed culture chemostat system was used to demonstrate that the pH generated from carbohydrate metabolism, rather than carbohydrate availability per se, was responsible for the shifts observed in the oral microflora which are associated with high carbohydrate diets and the development of dental caries. The aim of this study was to determine more accurately the microbially generated pH at which such shifts occurred. Nine oral bacteria were grown in three independent chemostats, and pulsed with glucose on 10 consecutive days. In one chemostat, pH control was discontinued for 6 h, and the pH fall was restricted to a minimum value of pH 5.5; the pH fall was arrested in the other two chemostats at either pH 5.0, or at pH 4.5. When the pH was allowed to fall, the numbers and proportions of Streptococcus mutans and Lactobacillus rhamnosus increased; this increase was directly related to the magnitude of the pH fall. Veillonella dispar was the most numerous organism following all glucose pulsing regimes, especially at low pH. The increase in proportions of acidogenic bacteria was accompanied by a fall in the proportions of acid-sensitive species (Fusobacterium nucleatum, Prevotella nigrescens, Streptococcus gordonii and Streptococcus oralis). The counts of these species were relatively stable between pH 5.5 and 4.5, but were markedly reduced when the pH fell below pH 4.5; Neisseria subflava could not persist in the culture at pH 4.5 or below. The data suggest that the disruption of communities associated with glucose metabolism and low pH can be explained in terms of a two-stage process. A fall in pH to a value between pH 5.5 and 4.5 may allow the enrichment of potentially cariogenic species, whilst permitting species associated with health to remain relatively unaffected. A further reduction in pH (<pH 4.5) may not only enhance the competitiveness of odontopathogens, but inhibit the growth and metabolism of non-caries-associated species. The results also indicate that species other that mutans streptococci or lactobacilli are competitive at pH values low enough to demineralise enamel, and thus suggest that a broader range of micro-organisms may be associated with caries initiation.