Influence of ionic strength and substratum hydrophobicity on the co-adhesion of oral microbial pairs

Co-adhesion between oral microbial pairs (i.e. adhesion of a planktonic microorganism to a sessile organism adhering to a substratum surface) has been described as a highly specific interaction, mediated by stereochemical groups on the interacting microbial cell surfaces, and also as a non-specific, critical colloid-chemical interaction. In a colloid-chemical approach, microbial co-adhesion is considered as an interplay between, amongst others, hydrophobic and electrostatic interactions. The aim of this paper was to determine the influence of ionic strength on the co-adhesion of Streptococcus oralis 34 to either Actinomyces naeslundii T14V-J1 or its mutant strain 5951 adhering to glass in a parallel-plate flow chamber. To this end, the ionic strength of the suspension was varied by the addition of KCl. Another aim was to investigate whether substratum hydrophobicity affected the co-adhesion between the organisms by allowing the sessile organisms (in this case the actinomyces) to adhere either to hydrophilic or to hydrophobic, dimethyldichlorosilane (DDS)-coated glass. The kinetics of co-adhesion of S. oralis 34 to the actinomyces decreased with increasing ionic strength, expressed as the ratio, chi, between the local and non-local initial deposition rates of the streptococci in the vicinity of, or far away from, the adhering actinomyces, respectively. In a stationary end-point of co-adhesion, ionic strength appeared not to be a determinant factor for the co-adhesion of S. oralis 34 with A. naeslundii 5951, either when the actinomyces were adhering to hydrophilic glass or to hydrophobic, DDS-coated glass. However, for S. oralis 34 co-adhering in a stationary end-point with A. naeslundii T14V-J1 in the high-ionic-strength (250 mM KCl) suspension, co-adhesion was far less on hydrophobic, DDS-coated glass than on hydrophilic glass. It is possible that the hydrophobic fibrils on A. naeslundii T14V-J1 bearing the lectin responsible for co-adhesion were immobilized in the latter case by adsorption to the hydrophobic substratum, making them less available for interaction with the streptococci.