Previous studies of metal adsorption onto bacteria have identified carboxyl and phosphoryl functional groups as important metal binding sites within bacterial cell envelopes, but many of these studies were conducted only under relatively high, environmentally unrealistic metal-loading conditions. Recently, low, but significant, concentrations of high-affinity sulfhydryl binding sites have been discovered on bacterial cell envelopes, and these sites have the potential to dominate metal binding mechanisms under the low metal loading conditions prevalent in most natural settings. In this talk, I will present a compilation of these recent studies. First, a novel approach was developed to estimate the concentration and acidity constants of sulfhydryl sites. The experiments involved the use of selective blocking of sulfhydryl sites with the monobromo(trimethylammonio)bimane bromide (qBBr) molecule, coupled with potentiometric titration measurements of blocked and un-blocked bacterial samples. Our results suggest that sulfhydryl sites are present in relatively low concentrations over a wide range of bacterial diversity, but that their concentrations are high enough to control the binding of metals onto bacteria under low metal-loading conditions. Second, we demonstrate that a single adsorption reaction can not account for Cd adsorption as a function of metal loading, and we use the data, in conjunction with EXAFS constraints, to constrain stability constants for Cd surface complexes involving both the high- and low-affinity sites. Lastly, we use a qBBr approach to determine if sulfhydryl sites exist within bacterial extracellular polymeric substances (EPS). The sulfhydryl concentrations within bacterial biomass samples before and after removing the EPS were measured by coupling selective site-blocking, potentiometric titration and surface complexation modeling. The results from this study suggest that the sulfhydryl concentrations within the EPS vary significantly from one bacterial species to another, with significant concentrations of sulfhydryl sites present in the EPS from some species, and none in others. Together, these studies demonstrate that metal-sulfhydryl binding can represent a dominant mechanism responsible for metal adsorption onto bacterial cell envelopes under low metal loading conditions, and therefore can play an important role in the transport, bioavailability and transformation of chalcophile metals in natural and engineered environments.

Characterizing high affinity binding sites within bacterial cell envelopes and on bacterial EPS molecules

249th National Meeting (2015)

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