January 2004

Superbug Survives Radiation and Imperviously Eats Waste

Conan the Bacterium
A can of spoiled meat and nuclear waste may appear to have little in common, but the microbe Deinococcus radiodurans finds both environments rather cozy. Scientists hope this organism’s ability to withstand massive doses of radiation will make it a useful tool for toxic-site remediation.

Although scientists now find it in many different soil and water sites around the world, D. radiodurans was not identified until 1956. It was isolated from a can of ground beef that had been radiation sterilized but had spoiled nonetheless. Perhaps because it can efficiently repair radiation breakage of its own DNA, D. radiodurans can endure 1.5 million rads of radiation, a dose 3,000 times higher than would kill organisms from microbes to humans. Scientists are unsure how this resistance evolved, although they suspect it may be a side effect of the microbe’s ability to survive periods of severe dehydration, which also fragments DNA.

Recognition of D. radiodurans’ resistance to radiation led DOE Microbial Genome Program (MGP) managers to believe the microbe could be useful in cleaning up mixed-waste sites contaminated with toxic chemicals as well as radiation. They began to fund projects to decipher the microbe’s genome and alter it to detoxify the most common chemical contaminants at these sites. Such detoxification functions might include concentrating heavy metals and breaking down organic solvents such as trichlorethylene.

Genetic Enhancements
Cleanup of toxic sites created by improper disposal of nuclear wastes presents a massive global challenge requiring innovative remediation approaches. In Nature Biotechnology, DOE grantees Michael Daly and Kenneth Minton (Uniformed Services University for the Health Sciences in Bethesda, Maryland) described a first step toward enhancing the D. radiodurans genome to make it valuable for toxic-site cleanup. The work also was featured in a four-page “Conan the Bacterium” article in the July-August, 1998, issue of The Sciences, the magazine of the New York Academy of Sciences.

In the Nature Biotechnology article, Daly and Minton reported successfully altering the microbe’s genome. This was accomplished by first fusing a gene encoding toluene dioxygenase (an enzyme that degrades the organic contaminant toluene) to a D. radiodurans promoter (a site that activates the gene). This DNA was then inserted into one of the bacterium’s chromosomes. The resulting recombinant bacterium is capable of degrading toluene and other organic compounds in a high-radiation environment. It also is tolerant of toluene and trichloroethylene’s solvent effects at levels exceeding those of many radioactive waste sites.