A quantitative study of calcium binding by isolated streptococcal cell walls and lipoteichoic acid: comparison with whole cells.

Calcium-binding by surface components of oral bacteria may have important effects on remineralization/demineralization phenomena and plaque cohesion. Additionally, some species export large quantities of lipoteichoic acid, possibly as a protective measure. Measurement of calcium-binding can facilitate prediction of how this will effectively buffer plaque fluid calcium concentration and affect these processes. Using equilibrium dialysis, we measured calcium-binding capacities and affinities at pH 7.0 in isolated cell walls of Streptococcus downei, S. sanguis, and purified lipoteichoic acid (LTA) of S. sanguis. Mean binding capacities were: 56.5 mumol Ca/g wet weight for S. downei cell walls and 47.2 mumol Ca/g wet weight for S. sanguis cell walls, and 1.11 mol Ca/mol LTA phosphate were found. Mean dissociation constants (mmol/L) for cell wall calcium binding were 2.16 mmol/L (S. downei) and 2.69 mmol/L (S. sanguis). These constants were not significantly different from those for whole cells of the same species (Rose et al., 1993), but the dissociation constant for LTA (7.82 mmol/L) was significantly higher and suggested a different mode of binding. At neutral pH, at the known calcium concentration of plaque fluid, whole cells and cell walls are likely to be completely saturated with calcium, whereas free LTA is only 30% saturated. The large amounts of LTA exported by some sucrose-grown streptococci may therefore act as a calcium buffer and so protect the organisms against high local concentrations of calcium produced during demineralization.