Metal binding onto bacterial cells and onto EPS molecules can control metal bioavailability and hence the extent and kinetics of bacterially-mediated reactions involving those metals. Although not the most abundant binding sites on bacterial surfaces and on EPS, thiol sites control the binding of chalcophile elements under environmentally-relevant metal loading conditions. The concentration and distribution of thiol sites between cell surfaces and EPS can control the behavior and fate these metals. For example, we report results that demonstrate that Se and Cd bioavailabilities to bacteria are controlled by binding onto thiol sites on bacterial surface and EPS molecules. These results demonstrate that thiol binding sites on bacterial surfaces and EPS molecules can play an important role in controlling the rate and extent of metal toxicity and metal oxidation/reduction by bacterial cells, and hence it is crucial to identify the hosts of these thiol groups and the controls on their abundance within bacterial biomass. Bacterial surface proteins that contain the amino acid cysteine may represent the dominant hosts of bacterial surface thiol sites, and proteomics represents a powerful tool to elucidate the location and origins of these sites. For example, sequence analysis indicates that B. subtilis cell wall binding proteins contain very few cysteine residues, but its EPS contains over 600 proteins that are distinct from cell wall binding proteins, and many of these are highly enriched in cysteine and originate in the cytoplasm. We hypothesize that during cell death, intracellular enzymes are released from the cell and the protein detritus subsequently adsorbs to other cells, and we report results that show that, at least for B. subtilis, EPS thiol binding sites are associated with detrital proteins that originate from decaying biomass and are adsorbed to the cell surface. Conversely, the distribution of thiol-bearing proteins between the S. oneidensis surface and its EPS matrix is opposite that observed for B. subtilis and P. putida, and may reflect different physiologic roles for chalcophile metals between bacterial species. The results of this proteomics analysis demonstrate the potential to identify the range of possible protein hosts for metal binding thiol sites, and the approach represents a means for predicting the concentration and distribution of thiol metal binding sites on bacterial cells and EPS molecules.

Role of bacterial surface and EPS sulfhydryl binding sites in controlling the rate and extent of metal bioavailability to bacteria

ACS Spring 2022

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