Oscillational motion properties of bacteria and polystyrene particles on a positively polarized substrate surface

The oscillational motion of bacteria and non-biological particles on a positively polarized substrate surface were investigated in this study using several bacterial species (Staphylococcus epidermidis ATCC12228 and Pseudomonas aeruginosa PA14) and polystyrene particles (modified with sulfate or carboxylate) that have different cell/particle size, surface potential, surface ionizable functional group, and surface appendage with respect to the mean square displacement (MSD) and motion trajectory. The attractive/repulsive interactions between the bacteria/particle and a positively polarized substrate surface are further discussed with the results of the motion analysis based on the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. As our major findings, all the bacterial species and particles showed oscillational motion, a kind of sub-diffusive motion that is more limited than the Brownian motion of the suspended bacteria/particles, on a positively polarized substrate surface. However, the motion properties among the bacteria/particles were found to differ in motion radius and MSD. As the size and negative surface potential of the bacteria/particle got smaller, the oscillational motion became more active, which may result from a decrease in attractive interactions such as van der Waals interaction and electrostatic attractive interaction. In the case in which some surface functional group (e.g., sulfate group) contributed to the formation of a strong Lewis acid-base interaction, the oscillational motion was significantly reduced regardless of the surface potential of the particle. The bacterial surface appendages were found to have no influence in explaining motion differences between the bacteria and non-biological particle.