Controls on the Concentration of Bacterial Cell Surface Sulfhydryl Sites: Effects of Inorganic Nutrients

Abstract

Understanding the controls on bacterial surface sulfhydryl site concentrations is crucial for modeling the effects of bacterial cells on the fate and transport of chalcophile metals in the environment. In this study, we investigate the effects of changing the concentrations of MgSO4, NaCl and NaNO3 in a growth medium on the concentration of cell surface sulfhydryl sites on the Gram-positive bacterial species Bacillus subtilis, as measured using a potentiometric titration approach. Our results show that the concentration of MgSO4 in a M9 minimal medium exerts a strong influence on bacterial surface sulfhydryl site concentrations under low MgSO4 concentrations but that this effect plateaus with increasing MgSO4 concentrations, indicating that a minimum amount of a sulfur source is required for bacteria to produce cell surface sulfhydryl sites. Although the sulfhydryl site concentration was below the detection limit on biomass samples that were grown in a M9 minimal medium containing 0.1 mM MgSO4, increasing the concentration of either NaCl or NaNO3 in the medium yielded biomass samples with measurable sulfhydryl site concentrations on the cell surface. In contrast, adding NaClO4 to the M9 medium did not increase cell surface sulfhydryl site concentrations. The NaCl and NaNO3 effects likely arise because increasing the concentration of NaCl or NaNO3 in a M9 minimal medium can promote the production of EPS by bacteria, and because the surface sulfhydryl sites of B. subtilis are located mostly on its EPS molecules. The results of this study suggest that bacterial nutrients and water chemistry composition are key controls on the abundance of bacterial surface sulfhydryl sites.