The role of physicochemical and structural surface properties in co-adhesion of microbial pairs in a parallel-plate flow chamber

Biofilm formation on solid surfaces involves interactions between planktonic organisms and the substratum as well as between flowing and already adhering organisms (''co-adhesion''). Biochemical studies have yielded information on the structural cell surface properties involved in interspecies binding between planktonic cells (''coaggregation''). No data are available, however, on the physicochemical mechanisms governing coaggregation and co-adhesion. It was the aim of this study to characterize the physicochemical surface properties of four Actinomyces naeslundii strains and three streptococcal strains, representing a selection of coaggregating and non-coaggregating oral microbial pairs, by microelectrophoresis, contact angles and X-ray photoelectron spectroscopy. Coaggregation appeared to be governed, at least partially, by electrostatic interactions. Analysis of the deposition kinetics using a parallel-plate flow chamber showed that adhering actinomyces act as local sinks for flowing streptococci. The initial local deposition rates of streptococci co-adhering to actinomyces are far higher than those calculated on the basis of convective-diffusion due to additional convective mass transport of streptococci to the adhering actinomyces protruding in the flow.