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
Hexavalent chromium [Cr (VI)] exists at the Superfund sites where it poses major health concerns to humans. Bacteria-mediated in situ reductive transformation of water soluble and toxic Cr(VI) to insoluble and less toxic Cr(III) in the form of Cr2O3 nanoparticle precipitates represents a promising method for bioremediation of Cr(VI) contamination. This research project focused on characterizing the molecular mechanism by which bacteria Shewanella oneidensis MR-1 reduces Cr (VI). Through the R01 project, the research team identified and demonstrated that that cell surface-exposed ctype cytochromes MtrC and OmcA of Shewanella play a critical role in the formation of Cr (III) particles on the bacterial cell surfaces by reducing Cr (VI) directly as Cr (VI) terminal reductases and indirectly as Fe (III) oxide terminal reductases to produce Fe (II) that in turn reduces Cr (VI).
Furthermore, the research team has applied the nanoscale AFM correlated SERS imaging microscopy (AFMCSIM), TEM, SEM, and EDEX imaging approaches along with other biological and electrochemical methods to investigate the bioremediation mechanism catalyzed by S. oneidensis MR-1. AFM-CSIM can simultaneously determine the locations of cytochromes and nano-sized metal particles as well as their oxidation states on the bacterial surfaces at nanometer-resolution.
Based on experimental results, the research team concluded:
- MtrC and OmcA are the terminal reductases used by S. oneidensis MR-1 for extracellular Cr(VI) reduction where OmcA is a predominant Cr(VI) reductase
- MtrC, OmcA, Cr(III) and hematite can be detected on S. oneidensis MR-1 cell surface by AFM-CSIM
- S. oneidensis MR-1 may also reduce Cr (VI) indirectly via MtrC/OmcA-reduced Fe (II).
In the course of this research project, the researchers have also closely correlated their research with their graduate education program to train the students in a broad range of microbiology, biological redox reactions, electrochemical analysis, and spectroscopic imaging analysis.