Atomic force microscopic corroboration of bond aging for adhesion of Streptococcus thermophilus to solid substrata

Initial bacterial adhesion is considered to be reversible, but over time the adhesive bond between a bacterium and a substratum surface may strengthen, turning the process into an irreversible state. Microbial desorption has been studied in situ in controlled flow devices as a function of the organisms resident time on the surface (J. Colloid Interface Sci. 164 (1994) 355). It appeared that desorption of Streptococcus thermophilus decreased strongly within approximately 50 s after initial adhesion due to bond aging. In this paper, bond aging between the S. thermophilus cell surface and the silicon nitride tip of an AFM (atomic force microscope) is corroborated microscopically and related to the macroscopic, residence time-dependent desorption of the organism under flow. AFM indicated bond strengthening between the tip and the cell surface within 100 s of contact, which is on the same order of magnitude as bond aging inferred from residence time-dependent desorption. Comparison of the interaction energies derived from AFM and macroscopic desorption indicate that bond strengthening arises as a result of multiple attachments of extracellular polymeric substances to a substratum surface.