Adsorption of a cationic polyelectrolyte on Escherichia coli bacteria:: 2.: Interactions between the bacterial surfaces covered with the polymer

Quaternized polyvinylpyridine (PVPQ) was used as a cationic polymer to destabilize an Escherichia coli bacterial suspension. The optical density and the fraction of free cells, obtained by light scattering measurements, were recorded as a function of the introduced polymer amount, as a way to monitor the stability of the suspension. The flocculation was almost complete for polymer dosages ranging from about 28 to 47 mg of carbon of PVPQ per dry g of bacteria. These dosages correspond to still negatively charged cells, as shown by xi potential measurements. At low polymer coverages, a less efficient flocculation is observed. At higher dosages, the suspension restabilizes. We interpret these results using our previous study on the adsorption of the polymer chains. We argue that the flocculation at low dosages is rendered possible by the strong inhomogeneities of charge on the bacterial surfaces because of the self-similar configuration of the adsorbed polymer layer and that the restabilization at large dosages is due to the small mesh size of the polymer network on the surface as well as to the Coulombic repulsion between the cells. The properties of the bacterial aggregates were investigated by light scattering. Destabilized suspensions produce aggregates with sizes decreasing as the quantity of adsorbed polymer increases. At the optimum of flocculation, the polydispersity of the aggregates is low, suggesting a diffusion-limited aggregation mechanism (DLA). The presence of the characteristic self-similar structure of DLA aggregates, with a fractal dimension on the order of 1.9, is suggested by some of the light diffusion experiments. On the other hand, at low coverages, that is, when only some regions on the surfaces are covered with polymers, the flocculation seems to obey a reaction-limited aggregation, with a large polydispersity in the size of the aggregates.