Competitive binding of calcium and magnesium to streptococcal lipoteichoic acid

Equilibrium dialysis was used to investigate the binding capacity and affinity of lipoteichoic acid (LTA) from the Gram-positive bacterium Streptococcus sanguis for calcium and magnesium by a competitive method. LTA was shown to bind approx. 1 mol of either calcium or magnesium per mole phosphate. Calcium and magnesium dissociation constants were found to be 8.39 +/- 0.31 mmol/l and 15.01 +/- 2.01 mmol/l respectively, indicating that S. sanguis LTA will preferentially bind calcium. LTA may act as a calcium buffer by reducing the free calcium concentration to which the cell is exposed. The capacity to produce large quantities of LTA could thus be as important as aciduricity in selection of species at caries-prone sites.