The biofilm matrix in sulfate-reducing bacterial biofilms: potential roles for electron mediators and large proteins

Abstract

Sulfate-reducing bacteria grow and form biofilms in soil and benthic environments across much of the Earth's surface. Formation of these prevalent biofilms requires the secretion of an extracellular polymeric substance (EPS) to allow the cells to stick together, as well as adhere to a surface. The specific interactions that occur between EPS components of an SRB biofilm are poorly understood. The data presented in this dissertation suggest the presence of two extracellular mechanisms utilized in these communities. The first mechanism was observed in a study altering the lactate (electron donor) and sulfate (electron acceptor) ratios to create limiting nutrient conditions in Desulfovibrio alaskensis G20 (G20) biofilms. G20 was grown under two conditions: electron donor limited (EDL) and electron acceptor limited (EAL) conditions. When grown on a 1018 carbon steel substrate, the G20 consumes all of the available lactate, and once limited, it turns to the high energy electrons in the Fe 0 for growth. Corrosion rates in the steel increased two fold compared to the EAL condition. Global metabolomic analysis revealed increased lumichrome levels under the EDL condition, which suggested higher flux through the riboflavin/FAD biosynthetic pathway. Previous research showed that synthetically adding riboflavin and FAD increases the corrosion rate of a SRB biofilm on 1018 carbon steel, and paired with these results, suggest G20 produces a flavin-based extracellular electron transfer molecule endogenously, and uses it to harvest high energy electrons from Fe 0 when limited for electron donor. The second mechanism was observed in Desulfovibrio vulgaris Hildenborough (DvH) biofilms grown on glass. Two proteins, DVU1012 and DVU1545 were found to be the most abundant extracellular peptides in a DvH biofilm. Single deletion strains for these proteins grew biofilm similar to the wild type strain, but a double deletion strain had decreased ability to form biofilm, demonstrating that at least one of the peptides must be present in order to form a biofilm. Exposure to increased shear force caused an large increase in wild-type biofilm biomass, yet eliminated the double mutant biofilm. These proteins are required for a DvH biofilm to respond to shear force.