The effects of heat inactivation, tortuosity, extracellular polyglucan and ion-exchange sites on the diffusion of [14C]-sucrose in human dental plaque residue in vitro.

Human dental plaque, accumulated for up to 24 h in vivo was packed into polyethylene tubing (0.5 mm, i.d.) by centrifugation at 5000 g for 15 min at 4 degrees C. [U-14C]-sucrose and carrier sucrose were diffused horizontally from one end and after 6 h, the [14C]-sucrose profile along the tube was quantified by liquid-scintillation counting and a diffusion coefficient was calculated. Compared with water, dental plaque significantly retarded the diffusion of sucrose. In live plaque, retardation of sucrose was less than in killed plaque, due to metabolism of sucrose to faster-diffusing species. Increasing the plaque tortuosity by increasing the centrifugal force used for packing reduced the diffusion rate further. Experimental conditions which increased the concentrations of water-insoluble, ethanol-precipitable extracellular glucan, EPG, in the plaque effected a reduction in the diffusion rate of sucrose. In contrast, an increase in EPG in batch cultures of Streptococcus mutans, serotype c, was associated with an increased rate of diffusion. The presence of ion-exchange sites did not affect the diffusion of sucrose in Sephadex gel models although the tortuosity of their pore structure resulted in a 6.5-fold reduction in the diffusion rate compared with water. Thus the retardation of the diffusion of sucrose in dental plaque is largely explained by the tortuosity of the available diffusion channels and the retardation of diffusion effected by EPG is probably of greater significance in older, thicker and more tightly-packed dental plaque.