Calcium and water diffusion in single-species model bacterial plaques

The diffusion of tritiated water and 45Ca through single-species model plaques was measured in a diaphragm cell. Using plaques of Streptococcus downei, the apparent diffusion coefficient for tritiated water showed a small but significant decrease between pH 7.0 and 5.0 with a smaller, non-significant decrease for calcium. These effects are attributed to possible plaque shrinkage at reduced pH, which overcomes effects due to charge-dependent changes in permeability. Effective diffusion coefficients for 45Ca obtained from lag-time measurements were much reduced at low carrier concentrations, demonstrating a binding which correlated with previous results from equilibrium dialysis. Strep. sanguis and Corynebacterium matruchotii showed few differences from Strep. downei, although the corynebacterium previously demonstrated stronger calcium binding. Permeability to water and calcium were slightly affected by pH, while effective diffusion of calcium depended on concentration. This may have important implications for the process of mineral loss in dental caries.