Electron Donor Effects on Bacterial Surface Sulfhydryl Site Concentrations

Abstract

In this study, we examined two effects on bacterial surface sulfhydryl site concentrations and distributions: (1) the effect of glucose concentration on the distribution of sulfhydryl sites between the bacterial cell surface and surface-associated extracellular polymeric substance (EPS) molecules, and (2) the effect of electron donor identity and concentration on sulfhydryl site concentrations on bacterial biomass. In each set of experiments, the total site concentration was measured using a potentiometric titration approach, and sulfhydryl site concentrations were determined using a site-specific blocking technique. The measurements were conducted with and without the removal of cellular EPS. For the first set of experiments, our results indicate that the two Gram-positive bacterial species, Bacillus subtilis, and Bacillus licheniformis, and one of the Gram-negative bacterial species studied, Pseudomonas putida, each have a greater concentration of sulfhydryl sites on their EPS molecules than is present on their cell surfaces, possibly serving to sequester toxic metals away from the cell surface. Conversely, for the Gram-negative bacterial species Shewanella oneidensis, the concentration of sulfhydryl sites on the cell surface is greater than that on its EPS molecules. S. oneidensis can gain metabolic energy through metal reduction, and hence enhancing the extent of metal binding to the cell wall through the formation of sulfhydryl binding sites may be more beneficial than the risk of metal toxicity. In the second set of experiments, increasing the concentration of two electron donors, pyruvate and glucose, in the growth medium of B. subtilis led to an increase in the percentage of total sites represented by sulfhydryl sites, but the concentration of the two other electron donors, glycerol and fumarate, had no effect on the percentage of sulfhydryl sites. Our results indicate that both the identity and the concentration of electron donors significantly influence the formation of sulfhydryl binding sites on bacterial cell surfaces. In addition, our results suggest that the total energy availability of a specific electron donor-bacterial species pairing affects both the ability of bacterial cells to produce sulfhydryl binding sites, and the distribution of those sites between the cell surface and its associated EPS molecules.