The Nature of Reactive Thiols on Bacterial Cell Envelopes and their Reactivity with Aqueous HG2+

Abstract

Thiol chemistry in natural systems is important to Hg biogeochemistry. The sorption of Hg to high affinity thiols of cell envelopes seems to influence Hg bioavailability and its subsequent methylation and reduction, since these transformations are primarily intracellular processes. Using a qBBr fluorescence titration method in water, I have observed that there are 24±2, 49±12, and 240±80 μmoles thiols per gram wet weight on the surface of Bacillus subtilis, Shewanella oneidensis MR-1, and Geobacter sulfurreducens, respectively, for the growth conditions presented. I also determined the thiol concentrations on a dry weight basis, as dry weight does not depend on the centrifugation speed used to harvest the cells: I observed 120±10, 300±70, and 1000±300 μmoles thiols per gram dry weight for B. subtilis, S. oneidensis MR-1, and G. sulfurreducens, respectively. These measurements support the hypothesis that the thiol density distribution directs Hg-(thiol)3 complexation on cell surfaces. It seems that clusters of three thiols in close proximity leads the nonmethylators B. subtilis and S. oneidensis MR-1 to form inert Hg-(thiol)3 complexes at low Hg concentrations (Mishra et al., unpublished), since they have lower average cell densities than G. sulfurreducens. The Hg-methylating G. sulfurreducens theoretically does not have such clusters on its cell envelope, as it does not form tridentate complexes (Mishra et al., unpublished) despite a relatively high average surface thiol density. To further our understanding of the thiol chemistry of Hg-contaminated natural systems, there is evidence that total thiols on the surface of S. oneidensis MR-1 will increase when the cells are grown in the presence of 250nM and 750nM Hg in Luria-Bertani (LB) broth. I have also observed that the fraction of Hg-thiol complexes that are Hg-(thiol)3 is largest when S. oneidensis MR-1 is grown with 100nM Hg2+ in LB broth (inoculated to about 5×106 cells/L) compared to the same concentration of cells grown with 0 nM, 50 nM, 250 nM, 500 nM, and 750nM Hg2+ in LB broth, which preliminarily suggests that at least some of the additional thiols are in clusters that are conducive to insoluble Hg-(thiol)3 complex formation.