Bacterial cell envelope and extracellular sulfhydryl binding sites: Their roles in metal binding and bioavailability

Although carboxyl and phosphoryl functional groups within the bacterial cell envelope and on bacterial extracellular polymeric substance (EPS) molecules are the most abundant metal binding sites, recent studies suggest that sulfhydryl sites control the binding of chalcophile and similar elements under environmentally-relevant metal loading conditions. The role of cell surface sulfhydryl sites in metal binding has been demonstrated unambiguously for Zn, Cd, Hg, Cu, Au, and Se. This review article summarizes our current understanding of the nature, concentration, and reactivity of these important metal binding sites, their distribution between the cell envelope and extractable EPS molecules, and their possible role in controlling bacterial bioavailability of some elements. The objective of the review is to summarize the relatively few studies that have focussed on bacterial sulfhydryl sites, and to identify areas in which future research may be most productive.