Effects of Surface Properties on the Charge Regulated Bioenergetic response of Attached Bacteria: Exploration within the Framework of the Chemiosmotic Theory

Microbial adhesion is critical to natural and engineered systems due to the ubiquitous presence of bacteria, their tendency to attach to biotic and abiotic surfaces and their ability to survive in habitats not suitable for most life. Many processes benefit from microbial adhesion, such as attached-growth wastewater treatment and symbiotic nitrogen fixation. In other scenarios, microbial adhesion is highly detrimental and examples include pathogenic biofilm infections on medical implants and devices and bio-corrosion of pipelines and ship hulls. Studies have demonstrated that the metabolic state of adhered bacteria can vary based on the physiochemical properties of the solid surface, but the reasons for this remained ambiguous until recent work by Hong and Brown. They proposed a hypothesis linking the charge regulation effect, which causes the local pH to vary as two surfaces with acid/base functional groups approach each other, and cellular bioenergetics, which stores energy in the form of a proton gradient across the bacterial cytoplasmic membrane. In their initial study, Hong and Brown proposed the hypothesis and demonstrated it for bacteria attachment to glass beads.Here, we demonstrate the validity of this hypothesis for a range of surfaces with different functional groups using experimental and modelling methods. Initial work focused on depicting that cellular bioenergetics of neutrophilic bacteria is influenced by changes in surface pH. Second, the effect of adhesion on metabolic activity of Escherichia coli was studied using a negatively-charged sand surface and a positively-charged goethite-coated sand surface. It was shown that the energy level of E.coli wasenhanced upon adhesion to the untreated sand and it was reduced upon adhesion to the coated sand, thus demonstrating the effect of solid surface functional groups on the metabolic activity of attached bacteria.The hypothesis was extended to study the impact of a range of acidic and basic surfaces on the bacterial metabolic activity making it possible to investigate a wide spectrum of attachment induced surface pH conditions. Adhesion experiments were performed with the Gram-negative E. coli and the Gram-positive Bacillus subtilis employing various surfaces in granular form. Surface characterization experiments and numerical modelling enabled the identification of the dissociation constants associated with functional groups on the bacterial surface and the solid surface which facilitated demonstration of a direct link between bacterial surface pH and cellular ATP levels.The results of the study indicated an overall relationship between solid surface functional group properties and bioenergetics of sessile bacteria. To summarize, upon adhesion to negatively-charged (acidic) surfaces, the charge regulated interface results in a proton-rich environment that stimulates ATP synthesis via chemiosmosis. The finite and rapid increase in ATP experimentally observed over the first 48 hours was followed by bacteria exhibiting an enhanced metabolic state through the course of the experiment. Attachment to basic surfaces results in a proton-deficit interface resulting in the depletion of intracellular ATP. The positive surfaces induced a declined metabolic state upon bacterial adhesion resulting in continual depletion of energy reserves over the experiment period.These findings can serve as the basis in the selection of surfaces and coatings to bring about a desired metabolic activity in attached bacteria based on requirements for the system at hand.