Energetics of the initial phase of adhesion of Streptococcus sanguis to hydroxylapatite

The initial adhesion of Streptococcus sanguis 10556 to artificial salivary pellicle and to bare hydroxylapatite was studied at several temperatures between 18 and 37 degrees C. When the natural logarithms of rate constants for adsorption and desorption were plotted against reciprocal temperatures in Arrhenius plots, curved lines were obtained, indicating that the thermodynamic quantities of enthalpy and entropy of activation were temperature dependent. For the bare hydroxylapatite system, the heat capacity (delta Cp = dH/dT) was large and negative. delta Cp was also negative for adhesion to saliva-coated hydroxylapatite, although its value was lower. Negative heat capacities, when coupled with favorable entropy, are often indicative of either electrostatic or hydrophobic interactions. When electrolyte (100 mM ammonium sulfate) was added to the cell-hydroxylapatite bead mixture, the rate and extent of adhesion were decreased. Addition of nonpolar p-dioxane (10% [vol/vol], final concentration) to the mixture enhanced binding. This suggests that electrostatic linkages participate in the primary adhesion of streptococci to both substrata. The strongly positive entropy values and the lesser temperature dependence of the saliva-coated hydroxylapatite system suggest that another entropy-driven process is imposed on the electrostatic linkages. This supports a role for hydrophobicity, suggesting that a combination of electrostatic and hydrophobic forces mediate the initial adhesion of S. sanguis to the salivary pellicle.