Soft particle analysis of bacterial cells and its interpretation of cell adhesion behaviors in terms of DLVO theory

The electrokinetic properties of two nitrifying strains, Nitrosomonas europaea and Nitrobacter winogradskyi, and three heterotrophic bacteria, Escherichia coli, Pseudomonas putida and Pseudomonas aeruginosa, were examined by electrophoretic mobility measurement and analyzed using the soft particle electrophoresis theory that is suitable for biological particles. The bacterial adhesion characteristics onto glass bead substratum were also evaluated by packed bed method. The mobility of the bacterial cells employed converged to a non-zero value as the ionic concentration increased, suggesting that the bacterial cells exhibited typical soft particle characteristics. Moreover, cell surface potentials based on the soft particle theory were lower than those estimated by the conventional Smoluchowski formula, i.e. zeta potential. Cell collision efficiencies onto glass beads (α₀) were largely dependent on interfacial interaction, although almost electrically neutral P. aeruginosa did not follow that trend. From a comparison of α₀ with DLVO interaction energy maximum (V_max), it was assumed that heterocoagulation between cell and substratum at primary minimum potential took place under V_max of 24-34 kT based on soft particle analysis. On the other hand, V_max predictions using the Smoluchowski theory gave 81-223 kT, which indicated the possibility of overestimating electrostatic repulsive forces by the conventional Smoluchowski theory. Thus, the application of this new electrophoresis theory to several kinds of bacterial cells has led to the revision of the interpretation of bacterial mobility data and provided a more detailed understanding of the bacterial adhesion phenomenon.