Effects of salivary film velocity on pH changes in an artificial plaque containing Streptococcus oralis, after exposure to sucrose

Results from a computer model suggest that following exposure of dental plaque to sucrose, the rate of clearance of acids from plaque into the overlying salivary film will be greatly retarded at low film velocities. This was investigated with an in vitro technique in which artificial plaque containing S. oralis cells was exposed to 10% sucrose for one min. The pH at the proximal (P) and distal (D) undersurfaces of the plaque (0.5 or 1.5 mm thick) was then monitored during the passage of a 0.1-mm-thick film of a sucrose-free solution over the surface. Over the range of salivary film velocities that have been estimated to occur in vivo (0.8-8 mm/min), lower minimum pH values and increased times for the pH to recover toward neutrality occurred at the lower salivary film velocity. Lower pH values were also reached with the 0.5- than with the 1.5-mm-thick plaque. P/D pH gradients, with a lower pH distally, developed at film velocities of 0.8 and 8 mm/min, and the gradients were much more pronounced at the lower velocity. No P/D pH gradients developed when the film velocity was 86.2 mm/min. Incorporation of dead S. oralis cells into the plaque at percentages up to 57% reduced the extent of the pH fall and prolonged the recovery of the pH toward neutrality. The results support the prediction that, other factors being equal, plaque located in regions of the mouth with low salivary film velocity will achieve pH values lower than those of plaque of identical dimensions and microbial composition located in areas where salivary film velocity is high.