FACTORS AFFECTING THE RESTING PH OF IN-VITRO HUMAN MICROCOSM DENTAL PLAQUE AND STREPTOCOCCUS-MUTANS BIOFILMS

The aim was to examine factors that potentially control the resting pH , defined as the pH unaffected by meals, of microcosm dental plaques a nd Streptococcus mutans biofilms under standard conditions, and to exa mine the effect of supplying urea at concentrations found intraorally. Microcosm plaques were cultured from plaque bacteria enriched saliva in an 'artificial mouth' with a continuous supply of a medium includin g 0.25% mucin [Basal Medium Mucin, (BMM), 3.6 ml/hr per plaque] and a periodic supply of sucrose. The steady-state resting pH was 6.4 (range +/- 0.1) in BMM containing no urea and supplied at the standard flowr ate. This isa robust property of the ecosystem. In one experiment with a replicated (n = 9) set of measurements, the resting pH was approx. pH 6.3, 6.4, 6.7 and 7.3 with 0, 1, 5 and 20 mmol/l urea in the BMM. T he magnitude of sucrose-and urea-induced pH responses was unaffected b y elevating the resting pH to produce parallel pH curves. The sucrose- induced pH curves were analogous to those classically reported by Step han that showed an association between caries activity and increasingl y acidic plaque pH responses to glucose. Stopping the BMM flow caused a pH rise, indicating continuing net alkali generation from BMM compon ents in the absence of a fluid flow. Strep. mutans monoculture biofilm s had an acidic resting pH of 5.0 to 5.3, which increased to 6.8 follo wing an adventitious superinfection by Bacillus cereus. It was conclud ed that the resting pH in plaque results from a delicate balance betwe en alkali and acid generation, which is in turn dependent both on the bacterial composition of the plaque and on the supply of substrates an d buffers from, and metabolite clearance into, flowing oral fluid. In vivo the resting pH will vary with site-specific changing saliva flows . Urea continuously supplied at concentrations normal for saliva and g ingival crevicular fluid can raise the resting pH of microcosm plaque by an amount that in vivo would probably be significant in reducing de ntal caries. (C) 1998 Elsevier Science Ltd. All rights reserved.