Physcial Sciences Division
Research Highlights

June 2009

"Breathing" Bacterium's Role in Remediation

Imaging and surface science give better understanding of electron exchange in contaminant conversion

Antibody recognition force microscopy (Ig-RFM) images of a live Shewanella oneidensis MR-1 cell growing anaerobically on hematite. A height image (bottom) and an Ig-atomic force microscopy (AFM) image (top) were collected simultaneously using a silicon nitride AFM tip that was functionalized with anti-OmcA antibody. In the bottom image, red color is high and blue color is low. The top image shows where OmcA molecules were (red and yellow color) or were not (blue color) detected using an anti-OmcA-functionalized AFM tip. The cell is approximately 1 μm by 2 μm. Enlarge Image.

Results:  A recent study of the contaminant-reducing bacterium Shewanella oneidensis provides the first evidence that the microbe maneuvers proteins within the bacterial cell into its outer membrane to contact metal directly. The proteins then bond with metal oxides, which the bacteria use to "breathe."

Bacteria such as Shewanella exchange electrons with minerals, affecting the migration of environmental contaminants and the biogeochemical cycling of iron and manganese in the subsurface. An international team of scientists determined the location, with nanoscale resolution, of two S. oneidensis MR-1 surface proteins, MtrC and OmcA. These are cytochromes, or surface-bound iron-containing proteins that facilitate this exchange between Shewanella and iron. The research was featured on the May 1 cover of Applied and Environmental Microbiology.

Why does it matter? Electron exchange is a basic function carried out by bacteria such as Shewanella. The research team's imaging studies offer a deeper understanding of the role MtrC and OmcA play in electron exchange. Understanding microbe-mineral exchange is of fundamental importance and may lead to advanced understanding of the fate and transport of radionuclide contaminants in subsurface environments, such as those found at the U.S. Department of Energy's Hanford Site. In turn, that will help in developing remediation strategies to protect humans and the environment.

Methods: The international research team used surface science and imaging capabilities at DOE's Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory. They used oxygen plasma-assisted molecular beam epitaxy to grow hematite thin films. Shewanella cells were allowed to attach to the thin films, where the iron in the hematite serves as an electron acceptor.

The researchers then used a dynamic force-scanning probe microscope for single-molecule force spectroscopy as well as antibody-recognition force microscopy (Ig-RFM) to map the locations of MtrC and OmcA on the surface of live Shewanella cells. A relatively new technique, Ig-RFM uses a nanometer-scale, flexible antibody-coated tip that is moved across a sample surface. When the antibody—in this case anti-MtrC or anti-OmcA—comes in contact with its binding partner, a measurable force is required to separate the two.

Force measurements indicate that MtrC is distributed rather uniformly on the bacterial surface, and OmcA is localized at the cell-mineral interface. Both cytochromes locate to the extracellular polymeric substance, which is made up of the secretions that help bacteria bind to surfaces.

Acknowledgments: The research was sponsored by DOE's Offices of Basic Energy Sciences and Biological and Environmental Research and the National Science Foundation.  Part of the research was performed as part of the EMSL Biogeochemistry Grand Challenge project sponsored by DOE-BER and located at PNNL.

The international team includes Liang Shi, Catherine Reardon, Grigoriy Pinchuk and Timothy Droubay, all PNNL; lead authors Brian Lower and Steven Lower, The Ohio State University; Ruchierej Youngsunthon, Corning Incorporated; Linda Wildling, Johannes Kepler University of Linz, Austria; Nicholas Wigginton, Ecole Polytechnique Fédérale de Lausanne, Switzerland; and Jean-François Boily, Umeå University, Sweden.

Read More

• "Tricking toxins" (link to the PDF) by reporter Spencer Hunt, The Columbus Dispatch April 5, 2009.
• "Soil Bacteria Thrive on Toxic Chemicals" by reporter John Hoffman, Philadelphia Science and Tech News Examiner, March 24, 2009.

Related PNNL Highlights

• Isolation and Validation of a Metal-Reducing Protein Complex from Shewanella Published in the Journal of Bacteriology
• Shewanella Uranium Reduction Role Described in PLoS Article

Reference: Lower BH, R Yongsunthon, L Shi, L Wildling, HJ Gruber, NS Wigginton, CL Reardon, GE Pinchuk, TC Droubay, J-F Boily and SK Lower. 2009. "Antibody-recognition force microscopy shows that outer membrane cytochromes OmcA and MtrC are expressed on the exterior surface of Shewanella oneidensis MR-1." Applied and Environmental Microbiology 75(9): 2931-2935.