The effect of electric fields on bacterial attachment to conductive surfaces

Abstract

Bacterial adhesion has been studied in various conditions. The bactericidal effect of strong electric voltages has been mainly tested within the context of preventing device-related infections in hospital environments. Little evidence is found in the literature on the mechanism of bacterial deposition as a function of an applied electric field. In this study, we assumed that an electric field, which was applied perpendicularly to the flow of a bacterial suspension, might impact the electrostatic energy barrier between the negatively charged bacterial cell and the positively charged electrode substrate. Experiments were carried out with a QCM-D electro-chemical module which allowed monitoring the adhered mass as well as the dissipation factor, while a bacterial suspension was passed through the module in a diffusion dominated flow regime and while an electric field was applied perpendicularly to the flow of the suspension. A Pseudomonas fluorescens strain was used in order to detect and estimate any bacterial adhesion on the electrode surface. While the results confirmed that a decrease of the recorded resonance frequency was in direct proportion to the amount of adhered bacteria (estimated by a visual control of the bacterial signal on the electrode sensor at the end of each experiment), the change in the resonance frequency as a function of the applied electric potential was rather counter-intuitive: with negative potential the rate of bacterial adhesion was faster and steadier than with a positive electric potential, when the bacterial rate of adhesion tended to level off. Moreover, analyses of the measured dissipation to the measured frequency shifts (ΔD/Δf) during the deposition experiment, which indicate the fluidic character of the adhered layer of bacteria, revealed that with negative electric potential the bacterial cells were rigidly connected, while with positive electric potentials the cells were rather loosely bound to the electrode. The measured data suggested that the applied electric potential might have an impact on the conformation of the surface appendages on the bacterial cell surface of P. fluorescens, causing a state of steric repulsion with an extended conformation induced by positive electric potentials, and a state of entangled conformations induced by negative potentials, which allowed the cells to overcome the electrostatic energy barrier.