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 import ant effects on remineralization/demineralization phenomena and plaque cohesion. Additionally, some species export large quantities of lipote ichoic 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 e quilibrium dialysis, we measured calcium-binding capacities and affini ties at pH 7.0 in isolated cell walls of Streptococcus downei, S. sang uis, and purified lipoteichoic acid (LTA) of S. sanguis. Mean binding capacities were: 56.5 mu mol Ca/g wet weight for S. downei cell walls and 47.2 mu mol Ca/g wet weight for S. sanguis cell walls, and 1.11 mo l 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), bu t the dissociation constant for LTA (7.82 mmol/L) was significantly hi gher and suggested a different mode of binding. At neutral pH, at the known calcium concentration of plaque fluid, whole cells and cell wall s are likely to be completely saturated with calcium, whereas free LTA is only 30% saturated. The large amounts of LTA exported by some sucr ose-grown streptococci may therefore act as a calcium buffer and so pr otect the organisms against high local concentrations of calcium produ ced during demineralization.