Role of acid–base interactions on the adhesion of oral streptococci and actinomyces to hexadecane and chloroform—influence of divalent cations and comparison between free energies of partitioning and free energies obtained by extended DLVO analysis

Abstract

Calcium is an abundantly present, divalent cation in the oral cavity and plays a crucial role in the adhesion of oral microorganisms to tooth surfaces as well as in coaggregation and coadhesion among the oral microflora. The aim of this study was to determine the effects of divalent cation (Ca 2+, Mg 2+, Ba 2+) adsorption on the adhesion of two actinomyces and two streptococcal strains to hexadecane (MATH) and chloroform (MATS) in order to detect changes in acid–base character of the cell surfaces. Initial removal rates of the organisms by hexadecane, lacking an acid–base interaction with the organisms, were always smaller than those by chloroform. Furthermore, adsorption of divalent cations generally increased the initial removal rates of the microorganisms, but no statistically significant differences among different cations were observed. Gibbs energies of partitioning calculated from the stationary end-point adhesion of the organisms ranged from −2 to −4 kT for adhesion to hexadecane and were about twofold more negative for adhesion to chloroform. Contact angles on lawns of microorganisms with and without adsorbed divalent cations were similar. Zeta potentials of all microorganisms were slightly negative under the conditions of MATH and MATS and became only 4 mV more positive upon divalent cation adsorption. Hexadecane had a zeta potentials of −21 mV in the potassium phosphate solution used, which became 13 mV less negative upon Ca 2+ adsorption. An extended DLVO approach of microbial adhesion to hexadecane, based on microbial contact angles and zeta potentials, taking into account Lifshitz–van der Waals, acid–base and electrostatic interactions did not show any potential energy barrier and demonstrated a deep primary interaction minimum at close approach due to acid–base attraction. As the Gibbs energy of partioning was only −2 to −4 kT, it is concluded that for the collection of organisms studied here, the final contactable surface area is small and structural features on the cell surfaces like fibrils and fimbriae, maintain a distance of ca. 10–15 nm between the hexadecane and the overall cell surface and therewith prevent acid–base interactions to become operative to a significant extend. Furthermore, from the lack of influence of divalent cations on macroscopic cell surface contact angles and zeta potentials, it is suggested that cation adsorption is minor and localized to the fibrils and fimbriae.