Role of lactobacillus cell surface hydrophobicity as probed by AFM in adhesion to surfaces at low and high ionic strength

The S-layer present at the outermost cell surface of some lactobacillus species is known to convey hydrophobicity to the lactobacillus cell surface. Yet, it is commonly found that adhesion of lactobacilli to solid substrata does not proceed according to expectations based on cell surface hydrophobicity. In this paper, the role of cell surface hydrophobicity of two lactobacillus strains with and without a surface layer protein (SLP) layer has been investigated with regard to their adhesion to hydrophobically or hydrophilically functionalized glass surfaces under well-defined flow conditions and in low and high ionic strength suspensions. Similarly, the interaction of the lactobacilli with similarly functionalized atomic force microscope (AFM) tips was measured. In a low ionic strength suspension, both lactobacillus strains show higher initial deposition rates to hydrophobic glass than to hydrophilic glass, whereas in a high ionic strength suspension no clear influence of cell surface hydrophobicity on adhesion is observed. Independent of ionic strength, however, AFM detects stronger interaction forces when both bacteria and tip are hydrophobic or hydrophilic than when bacteria and tip have opposite hydrophobicities. This suggest that the interaction develops in a different way when a bacterium is forced into contact with the tip surface, like in AFM, as compared with contacts developing between a cell surface and a macroscopic substratum under flow. In addition, the distance dependence of the total Gibbs energy of interaction could only be qualitatively correlated with bacterial deposition and desorption in the parallel plate flow chamber.