Deinococcus radiodurans Kevin Walker
In 1956 scientists at the Oregon Agricultural Experiment Station were testing the effectiveness of using radiation to sterilize canned foods. In this experiment cans of food were being bombarded with enough gamma radiation to kill any known form of life. The results of this experiment astounded the researchers there – a tin of meat spoiled following exposure to such high doses of energy. Arthur Anderson was the first to notice, as well as isolate, the red bacterium later named Deinococcus radiodurans, meaning “strange berry that withstands radiation.”
D. radiodurans is a gram-positive, red-pigmented bacterium that typically grows in a tetrad structure. It is a nonsporulating and strictly aerobic bacterium, but it has been known to survive in vacuum conditions. D. radiodurans is a chemoorganotroph with an optimal growth temperature of 25º – 35º C; however, it is very resistant to extreme temperatures. The bacterium has been isolated from numerous locations, including elephant dung and the granite of Antarctica’s dry valleys. The natural habitat has not been determined.
D. radiodurans has been the focus of significant study since that day of discovery more than fifty years ago. Its ability to withstand harsh conditions such as cold, heat, vacuum, dehydration, and acid are certainly impressive. These abilities are more than sufficient to classify this bacterium as an extremophile. However, it is the unparalleled resistance of D. radiodurans to enormous doses of radiation that has captivated scientists for so long. Five gray (Gy) is a lethal dose of radiation to humans, while 60 Gy is enough to kill all the cells in an Escherichia coli sample. However, D. radiodurans can withstand 5,000 Gy with no loss of viability, making it the most radiation resistant organism known. When exposed to 15,000 Gy this bacterium will maintain 37% viability, a dosage of radiation that is more than 3000 times what a human can withstand.
The origin of such extreme radiation resistance has been the topic of much speculation and research. Normal levels of background radiation on earth are around 0.4 mGy per year, with the highest known levels being 175 mGy per year in parts of Brazil. This raises the question of why a bacterium would evolve to withstand radiation doses of 10,000 Gy when exposure to such high levels is very unlikely. Some scientists pose a theory that D. radiodurans may have originated in outer space where cosmic radiation is very significant. The bacterium could then have been brought to earth by way of a meteorite. Other scientists argue this is an unlikely possibility, contending that the similarities of D. radiodurans to other prokaryotes suggest a terrestrial origin. Studies have indicated that the effects of dehydration on DNA are very similar to the effects of radiation. Although normal strains of D. radiodurans are resistant to both desiccation and radiation, mutated strains have become susceptible to damage from both conditions. This has led some scientists to conclude that D. radiodurans originally evolved to withstand long periods without water. The radiation resistance that has come to define this bacterium may actually have been a byproduct of the mechanism for surviving dehydration.
The mechanism by which D. radiodurans withstands such extreme levels of radiation is still not completely understood. It is clear that this bacterium has exceptional DNA repair mechanisms. Studies have shown that after exposure to high doses of radiation, when the genome has been greatly fragmented, D. radiodurans is able to restore its DNA within a matter of hours. The process is believed to take place in two distinct steps; first the bacterium goes through a process of single strand annealing to recombine the complimentary strands that have separated. Once this has been done a RecA protein recombines double strand breaks in the DNA through homologous recombination. This repair system works with precise accuracy ensuring that the bacterium is able to survive. In addition to its rapid repair mechanisms D. radiodurans typically contains 4-10 copies of its genes. Scientists believe that this characteristic allows RecA protein to use an intact strand of DNA as a template for restoring the fragmented chromosome. Also, this bacterium has a toroidal-shaped DNA molecule that may contribute to its radiation resistance by aligning identical copies of its genome near each other. This would allow for much faster repair of the fragmented DNA.
The rapid progression in genetic engineering has already allowed scientists to utilize the unique characteristics of D. radiodurans. A mercuric reductase gene found in E. coli has been cloned into strains of D. radiodurans, thus allowing for the detoxification of ionic mercury in some radioactive waste. The radiation resistance of this organism is also evoking high hopes in the area of radioactive waste cleanup. If this bacterium can be genetically engineered to metabolize heavy metals such as plutonium and uranium, a very difficult task, then effective cleanup may be possible. Prior to the techniques of genetic engineering this would have seemed impossible – no organism could live long enough under high doses of radiation to consume such materials. A modified strain of D. radiodurans may present a solution to this problem.
The discovery of this fascinating microbe has been very important in the world of microbiology. The current uses of D. radiodurans are significant, but the possible applications for such a bacterium are extraordinary. The seemingly impossible abilities of this organism can only lead to an exciting search for other microbes that defy the impossible.
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4. “Photoinactivation of Deinococcus radiodurans: An unusual gram-positive Microorganism.” Find Articles. 2007. 16 Feb 2007. <http://www.findarticles.com/p/articles/mi_qa3931/is_199904/ai_n8845879>
5. “Deinococcus radiodurans - a radiation-resistant bacterium.” Uniformed Services University of the Health Sciences. 2006. 16 Feb 2007. <http://www.usuhs.mil/pat/deinococcus/index_20.htm>
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.
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