Microrheological aspects of adhesion of Escherichia coli on glass

The adhesion of both live and fixed bacteria (Escherichia coli) on glass has been studied under well-defined hydrodynamic conditions, created in an impinging jet apparatus. With this technique one can accurately measure the initial deposition rate j₀ on the surface, the average lifetime of a bacterium on the surface, τ_esc and the surface area blocked per deposited bacterium, normalized by its projected area, γ. The experimental results are compared to theoretical results for equivalent spheres. It is found that near the stagnation point the deposition rate j₀ is mainly controlled by convective diffusive transport which, for rod-shaped Escherichia coli, with an axis ratio of about 2, is found to be equal to that for spheres. No differences in j₀ and τ_esc were found between live and fixed bacteria at low flow rates. At high flow rates fixed bacteria adhered to the surface at a slower rate. In both systems j₀ was found to decrease suddenly at a distance of about 150 μm from the stagnation point, in contrast to systems of spherical particles for which j₀ is uniform over the surface. Most likely this is due to the rotation of the rod-shaped particles, which vary their distance to the surface periodically with time. The main difference between live and fixed bacteria, besides different deposition rates in strong flows, is that γ is about 30% larger for fixed bacteria than for live ones, resulting in a much lower final coverage for fixed bacteria. These results imply a larger repulsion between fixed bacteria than between living ones. From detachment experiments we can conclude that not all bacteria stick to the surface with the same bond strength. The variation in the bond strength is due to the aging of the bonds between the bacteria and the surface. The average bond strength corresponds to an energy of about 13-15 kT.