Investigation of calcium-binding sites on the surfaces of selected Gram-positive oral organisms

Abstract

Dental plaque is rich in anionic groups with a high calcium-binding capacity which may affect mineral dynamics at the tooth surface. The two major calcium-binding sites on Gram-positive cell surfaces are carboxylate groups (in proteins and peptidoglycan cross-links) and phosphate groups (in lipoteichoic and teichoic acid). Equilibrium dialysis was used to measure calcium-binding capacities of whole cells and purified cell-wall material (CWM) from Streptococcus mutans R9, Strep. oralis EF186, Strep. gordonii NCTC 7865, Strep. downei NCTC 11391, Actinomyces naeslundii WVU627 and Lactobacillus casei AC413. This material was stripped of phosphate (PS-CWM) and treated to mask carboxylate groups (CM-CWM). Whole-cell calcium-binding capacities ranged from 240 ( Strep. downei) to 50 ( L. casei) μ mol/g (dry wt). Differences in CWM, PS-CWM and CM-CWM calcium-binding capacities demonstrated the greater importance of phosphate in comparison with carboxylate groups in cell calcium binding. These data indicate that, in streptococci, calcium binding is predominantly phosphate group-based, especially in the teichoic acid-containing Strep. oralis. In the other species tested, calcium binding is predominantly carboxylate group-based.