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

Abstract

The initial adhesion of Streptococcus sanguis 10556 to artificial salivary pellicle and to bare hydroxylapatite was studied at several temperatures between 18 and 37C. 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 (ACp = dH/dT) was large and negative. ACp 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.Significant advances have recently been made in the description of cellular adhesion mechanisms. Mammalian (23) and bacterial (6) cells have been shown to adhere through complex interactions with attractive and repulsive forces operating near substrata. Adhesion of streptococci to the salivary pellicle has long been recognized to be a complicated process. From the time of their eruption, teeth are constantly covered with a coating of salivary glycoproteins and proteins (pellicle). This pellicle is the substratum for oral bacterial attachment. Streptococcus sanguis seems to possess some specificity for the pellicle components since it reaches an equilibrium with the substratum. However, that specificity has been difficult to define, and it is believed that multiple forces govern adhesion (10)(11)(12). Kinetic studies suggest that S. sanguis passes through two distinct binding transitions (8). The first stage is a reversible one described by pseudofirst-order rate constants for adsorption and desorption. The cells bound in this equilibrium may then become more firmly attached and have a decreased propensity to desorb in the second phase of the reaction. The proposed mechanism can be represented as Cell + pelliclez±CP*--*CP 1 1 in which CP* represents the first, lower-affinity association between cell and pellicle. The rate constants governing this phase of the reaction are low, suggesting that the initial association is the rate-limiting one. Cells often adhere to solid surfaces in this general sequence. Marine pseudomonads (...

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