On relations between microscopic and macroscopic physicochemical properties of bacterial cell surfaces: an AFM study on Streptococcus mitis strains

Bacterial adhesion to substratum surfaces is determined by the combined action of a large number of different interactions, which have hitherto been inferred from macroscopic cell surface properties, such as electrostatic double-layer forces, hydrophobic interactions, hydrogen bonding, and steric interactions. The origin of these interactions arises from a molecular level, but nevertheless they have always been estimated from a macroscopic point of view. The macroscopic bacterial cell surface hydrophobicity, for instance, is commonly inferred from water contact angles on bacterial lawns, and cell surface charge is macroscopically determined by electrophoresis or titration methods. Although these macroscopic properties of bacteria have been demonstrated to correlate with bacterial adhesion of certain strains and species to substratum surfaces, a generalized physicochemical understanding of bacterial adhesion to surfaces does not yet exist. The introduction of the atomic force microscope (AFM) and its application to biological surfaces has offered new possibilities to obtain microscopic, physicochemical properties of bacterial cell surfaces. In this paper, a detailed analysis of the interaction forces between a silicon nitride AFM tip and the surface of nine different oral bacterial strains, Streptococcus mitis, was carried out. Interestingly, microscopic features of force-distance curves could be amalgamated in such a way that relations between microscopic cell surface properties and macroscopic cell surface properties were obtained, even though these relations were not fully understood.