Aim: To explore the Ecological Plaque Hypothesis for dental caries. To test modification of the microbiota of dental plaque microcosm biofilms by sucrose pulsing during growth in two different simulated oral fluids, and with a urea-induced plaque pH elevation. Methods: Plaque microcosm biofilms were cultured in an ‘artificial mouth’ with and without 6-min 5% w/v sucrose pulses every 8 h in an environment of continuously supplied saliva-like defined medium with mucin (DMM), or basal medium mucin (BMM, a high-peptone-yeast extract oral fluid analogue), and also in DMM + 10 mmol/l urea, with sucrose pulsing. Forty plaque species were quantified by checkerboard DNA:DNA hybridization analysis. Results: Sucrose pulsing extended rapid plaque growth in DMM and BMM, inducing major microbiota changes in DMM but not in BMM. In DMM, some streptococci and lactobacilli were unaffected while others implicated in caries, together with Candida albicans and Capnocytophaga gingivalis, increased. Aerobic, microaerophilic and major anaerobic species decreased. Elevation of the pH_max from 6.4 to 7.0 had almost no effect on the microbiota. BMM plaques were distinct from DMM plaques with particularly low levels of Candida albicans and Actinomyces. Conclusions: Modest sucrose exposure in a saliva-like environment causes profound changes in the developmental self-organization of plaque microcosms, supporting the Ecological Plaque Hypothesis. Nevertheless, there is significant stability in microbial composition with varying pH near neutrality. Increases in levels of specific bacteria in response to sucrose could be characteristic of organisms particularly important in caries.

No abstract

A reduced pool of calcium in dental plaque would be expected to increase the ability of plaque fluid to dissolve the underlying enamel when the pH falls during sugar exposure. We have examined the relationship between frequency of sugar application and Ca and P_i concentrations in artificial mouth plaque microcosm biofilms. Ten plaques were grown simultaneously from a human saliva inoculum using a continuous flow of simulated saliva, DMM, supplemented with either urea or glucose to modulate the resting pH. In addition the plaques received sucrose applications of varying frequency: 12-, 8-, 6-, or 4-hourly, or not at all. After 15 days the plaques were sampled by taking 4 full-thickness specimens of each, and acid-extractable Ca and P_i, and alkali-soluble protein and carbohydrate were determined. Ca and P_i concentrations were in a range comparable with those in human plaque, except in the DMM + urea plaque receiving no sucrose, when concentrations were higher. Plaque Ca concentration decreased significantly as sucrose application frequency increased. Increasing sucrose application frequency also reduced the protein, i.e. the cell biomass, content of the plaques and, in the case of DMM + urea plaques, increased the water-insoluble hexose content, presumably extracellular polysaccharide. Reduced biomass was partly due to the bulking of plaque with extracellular polysaccharide, but the marked effect of urea on polysaccharide formation is not understood. This study shows that increasing frequency of sugar application alters dental plaque by reducing its mineral protection capacity.

Biolog technology was applied to measure the metabolic similarity of plaque biofilm microcosms, which model the complex properties of dental plaque in vivo. The choice of Biolog plate, incubation time, and incubation conditions strongly influenced utilization profiles. For plaque biofilm microcosms, Biolog GP2 plates incubated anaerobically in an H 2 -free atmosphere gave the clearest profile. To test the application of the Biolog GP2 assay, plaque microcosms were developed under different nutrient conditions in which the frequency of sucrose application was varied. Cluster analysis of Biolog GP2 data from 10 microcosm biofilms correlated with sucrose frequency. Aciduric bacteria (Streptococcus mutans plus lactobacilli) predominated in the plaques receiving high-frequency sucrose applications. Agreement between the Biolog GP2 groupings with nutrient and compositional changes suggests that Biolog analysis is a valuable technique for analyzing the metabolic similarity of dental plaque biofilm microcosms and other high-nutrient or predominantly anaerobic ecosystems.

No abstract