Effect of ionic strength on initial interactions of Escherichia coli with surfaces, studied on-line by a novel quartz crystal microbalance technique

A novel quartz crystal microbalance (QCM) technique was used to study the a dhesion of nonfimbriated and fimbriated Escherichia coli mutant strains to hydrophilic and hydrophobic surfaces at different ionic strengths. This tec hnique enabled us to measure both frequency shifts (Delta f), i.e., the inc rease in mass on the surface, and dissipation shifts (Delta D), i.e., the v iscoelastic energy losses on the surface. Changes in the parameters measure d by the extended QCM technique reflect the dynamic character of the adhesi on process. We were able to show clear differences in the viscoelastic beha vior of fimbriated and nonfimbriated cells attached to surfaces. The intera ctions between bacterial cells and quartz crystal surfaces at various ionic strengths followed different trends, depending on the cell surface structu res in direct contact with the surface. While Delta f and Delta D per attac hed cell increased for nonfimbriated cells with increasing ionic strengths (particularly on hydrophobic surfaces), the adhesion of the fimbriated stra in caused only low-level frequency and dissipation shifts on both kinds of surfaces at all ionic strengths tested. We propose that nonfimbriated cells may get better contact with increasing ionic strengths due to an increased area of contact between the cell and the surface, whereas fimbriated cells seem to have a flexible contact with the surface at all ionic strengths te sted. The area of contact between fimbriated cells and the surface does not increase with increasing ionic strengths, but on hydrophobic surfaces each contact point seems to contribute relatively more to the total energy loss . Independent of ionic strength, attached cells undergo time-dependent inte ractions with the surface leading to increased contact area and viscoelasti c losses per cell, which may be due to the establishment of a more intimate contact between the cell and the surface. Hence, the extended QCM techniqu e provides new qualitative information about the direct contact of bacteria l cells to surfaces and the adhesion mechanisms involved.