Superfund Research Program

In Vivo Characterization of Bacteria-mediated Extracellular Reduction of Chromium

Project Leader: H. Peter Lu
Grant Number: R01ES017070
Funding Period: 2009-2011

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Final Progress Reports

Year:   2010 

Single-cell reduction of toxic Cr (VI) using a dissimilatory metal-reducing bacteria, Shewanella oneidensis MR-1, was investigated by correlated topographic-optical imaging, Raman spectroscopy, and scanning and transmission electron microscopes combined energy-dispersive X-ray spectroscopy (EDX). Shewanella oneidensis MR-1 reduced Cr (VI) to Cr₂O₃. The reduced Cr₂O₃ was observed as insoluble nanoparticles adsorbed on the cell surface as well as inside bacterial cells. The chemical nature of reduced Cr(III) was indentified by EDX imaging and Raman spectroscopy. The extracellular reduction of Cr(VII) was mediated by the outer membrane cytochromes MtrC and OmcA. Compared to that of the wild type, an in-frame deletion mutant lacking the genes encoding MtrC and/or OmcA lowered the reduction rate. While deletion of MtrC or OmcA had little impact on the localization of reduced Cr(III), deletion of both genes affected the extracellular localization of Cr(III), but not its intracellular localizations. In vitro characterization of purified MtrC and OmcA showed that both cytochromes reduced Cr(VI) with similar K_M, but different k_cat. Together, all these results consistently demonstrate that MtrC and OmcA are the terminal reductases that reduce Cr(VII) extracellularly. Moreover, results also indicated that direct microbial Cr (VI) reduction by MtrC and OmcA and Fe (II)-mediated Cr (VI) reduction mechanisms may operate concurrently.