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Deinococcus radiodurans has been labeled "the world's toughest microbe" by The Guinness Book of World Records because it is the most radiation resistant microorganism yet reported. This unique attribute may someday make this "Superbug" the microbe of choice for remediating highly radioactive waste.

This photomicrograph shows an image of unstained thin section of D. radiodurans cells that have reduced uranium. The D. radiodurans cells are approximately 2 microns in diameter and the reduced uranium appears as a dark fine-grained precipitate both internal and external to the cell and as small dots associated with the outer cell surface.

February 12—Scientists at the Department of Energy's Pacific Northwest National Laboratory (PNNL) recently reported that Deinococcus radiodurans, a microbe that holds promise for cleaning up dangerous contamination, can anaerobically (without oxygen) reduce radioactive and metal contaminants such as uranium, technetium, and chromium.

Until recently, D. radiodurans was believed to have a strictly aerobic metabolism (one that needs oxygen). Microbes that are able to live in anaerobic conditions and metabolize contaminated substances may prove to be valuable for cleaning up contaminated environments, which may be oxygen-deficient and provide little food for most organisms.

As part of DOE's Natural and Accelerated Bioremediation Research (NABIR) program, Jim Fredrickson and colleagues at PNNL and the Uniformed Services University of the Health Sciences in Bethesda, Maryland, discovered that D. radioduranscan enzymatically reduce radionuclides and metal contaminants in the absence of air. When provided a simple carbon and energy source, such as lactate, this microbe produced relatively insoluble and immobile forms of the contaminants.

DOE faces challenging cleanup problems at more than 18 facilities across the U.S. resulting from the production of weapons-grade nuclear materials between 1945 and 1986. Among the most common inorganic contaminants at these sites are the elements uranium, plutonium, technetium, and chromium. All of these are less mobile when reduced by microorganisms. However, some contaminated sediments and soils at DOE sites, particularly beneath leaking waste storage tanks, can have radiation levels that exceed those tolerated by most microorganisms. D. radiodurans can withstand radiation levels up to 1.5-million rads—500 rads is lethal to humans. It might be the only known microorganism that can survive such conditions and may someday be used to remediate highly radioactive waste.

What Makes "Superbug" so Tough?

The durability of this microorganism testifies that it is better able to repair its radiation-damaged DNA than other organisms. Dick Smith and colleagues at PNNL are conducting proteomics studies to understand the hardy biology of D. radiodurans—by examining the gene-to-protein processes of its cells. Proteomics is the study of proteins that are expressed by the genetic array of an organism.

The researchers used unique high magnetic field Fourier transform ion cyclotron resonance mass spectrometry at PNNL's Environmental Molecular Science Laboratory to identify the microbe's proteins. About half of all possible gene products that might be expressed by D. radiodurans were identified—a significantly higher fraction than has been detected for any other organism yet. Their methods allowed them to precisely measure the expression levels for all proteins in a single experiment. They are currently using the same approach to track changes in protein-expression patterns after D. radiodurans has been exposed to ionizing radiation.

The results from these studies will likely have important implications for the bioremediation of contaminated sites and within high-level radioactive waste storage tanks, where other metal-reducing microorganisms might not survive or be able to function. In such situations, D. radiodurans may be effective at reducing contaminants such as chromium and uranium in place, thus preventing their migration through soils and into ground water.

Microbes of the deinococci family appear to be widely distributed in soils and have been routinely isolated from organically rich as well as dry, nutrient-poor environments. Therefore, it is possible that deinococci capable of reducing metal and radionuclides may be native to some contaminated environments. To begin the useful process of reducing radioactive metals, D. radiodurans requires the presence of humic acids, which can be added if none occur naturally at a site.

Additional research is required to better understand the ecology of the deinococci and the potential for naturally occurring strains to reduce metals. These studies were supported by DOE's Office of Biological and Environmental Research, Life Sciences Division, and the Natural and Accelerated Bioremediation Research (NABIR) program.—James R. Weber

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