Deinococcus radiodurans Tim Lottman
In the past century, mankind has begun to develop a substantial understanding of his/her surroundings, which includes the subject of microbiology. However, as much as man has accomplished, he/she seems to have only scratched the surface of science in general. One such scratch, which changes scientific perspective almost daily, is that of microorganisms. The complexity and diversity associated with microorganisms make them a difficult area of research, classification and application. Of the many microorganisms discovered, one in particular has received a great deal of attention due to its extraordinary capabilities and seemingly endless potentials. This microorganism has been assigned the name Deinococcus radiodurans. To completely grasp the importance of this microorganism, one must delve into its history, explore its fascinating features and touch upon its human significance.
To begin, the historical roots of D. radiodurans are seeded in laboratory experiments. D. radiodurans were discovered in 1956 by Arthur W. Anderson at Oregon Agricultural Experiment Station in Corvallis. Anderson first noticed and later isolated the red bacteria from a can of ground meat that had spoiled despite having been sterilized with radiation in the megarad range. D. radiodurans, literally meaning "strange berry that withstands radiation," have since been classified as members of the family Deinococcaceae. It is wise to note that Deinococcaceae have the distinctive feature of being the most radiation-resistant of vegetative cells. The family Deinococcaceae consists of the genera Deinococcus and Deinobacter, with the major difference between these two genera being gram-positive and gram-negative, respectively. In addition, Deinococcaceae are aerobic, require very complex media for growth and produce pink to reddish colonies.
With the above serving as informative background reference, one may now examine the intriguing array of captivating features that D. radiodurans have to offer. Among the many characteristics of D. radiodurans, a few of the most noteworthy include an extreme resistance to genotoxic chemicals, oxidative damage, high levels of ionizing and ultraviolet radiation, and dehydration. The ability to survive such extreme environments is attributed to D. radiodurans ability to repair damaged chromosomes. It is known that heat, dehydration and radiation causes double-strand breaks in chromosomal DNA. D. radiodurans will repair these chromosome fragments, usually within 12-24 hours, using a two-system process with the latter being the most crucial method. Initially, D. radiodurans use a process called single-strand annealing to reconnect some chromosome fragments. Next, D. radiodurans use a process known as homologous recombination, where a modified yet efficient RecA protein patches double-strand breaks. RecA protein works by cutting usable DNA from another molecule and inserting it into the damaged strand. However, these repair methods alone are not unique to D. radiodurans, which therefore cannot account for its radiation resistance. The aforementioned statement has led scientists to propose the "Life Saver" hypothesis. The hypothesis states, that in order to speed homologous recombination, D. radiodurans align copies of its genome so that identical DNA sequences are near each other. This proposal is now entirely possible due to the verification that D. radiodurans genes come packaged in four distinct circular chromosomes, thus giving stacked loops of DNA and resembling a Life Saver. To add to the list of radiation protective traits, D. radiodurans also possess carotenoid pigments, oxygen toxicity defense enzymes, and a distinctive outer membrane. First, carotenoids, which cause red pigmentation, are thought to act as free radical scavengers, thus increasing resistance to DNA damage by hydroxyl radicals. Next, high levels of enzymes such as superoxide dismutase and catalase both play a role in effective defense mechanisms against oxygen toxicity. Finally, a cell wall forming three or more layers with complex outer membrane lipids and a thick peptidoglycan layer containing the amino acid omithine also serves to protect D. radiodurans from lethal doses of radiation.
Now that the intricacies of D. radiodurans have been covered, one may explore the possibilities for utilizing the microbe's special skills. At the top of the list for uses of D. radiodurans is bioremediation. Bioremediation is the strategy of using bacteria to feed on or simply degrade dangerous compounds. Although simple in theory, scientists have discovered microbes with an ability to metabolize one toxic substance, only to find that other compounds in a waste mix inhibit the bacterium's growth. D. radiodurans may offer a solution by providing the framework to create a versatile and efficient "superbug." The design of the superbug revolves around equipping D. radiodurans with genes imported from other bacterium already known to degrade dangerous compounds. Though organic pollutants such as trichloroethylene and toluene may be metabolized by specific microbes already known, no known bacterium can actually metabolize uranium, plutonium, and other heavy metals into harmless substances. However, some microbes do possess genes encoding proteins that immobilize metals with which they come in contact. By implementing these genes into the superbug, at least the spread of radioactive elements and other metals could be stifled until other cleanup strategies are available. So, although D. radiodurans may not provide a direct solution for waste treatment, it does provide scientists with a potential alternative.
To conclude, Deinococcus radiodurans is not just another microorganism to be quickly classified and overlooked. Instead, by understanding its phylogenic roots, characteristics, and potentials, one may easily recognize the impact that such a remarkable microbe could have in terms of human significance. Furthermore, a microbe with such tremendous capabilities should only fuel the fire to expand the understanding of all related fields of science. These great discoveries should only be small steps on the path to establishing explanations of our surroundings. Indeed, Deinococcus radiodurans have made a significant contribution to the field of microorganisms and have also provided many practical uses for society today.
Science News, Vol. 154, No. 24, December 12, 1998, p.376
Brock Biology of Microorganisms, 9th Ed., Madigan, Martinko, Parker, Prentice Hall, Upper Saddle River, NY, 2000, p.541
Battista, J.R. Against All Odd,-,: The Survival Strategies of Deinococcus radiodurans., 1997, p. 203-224.
The Sciences, July/August 1998, Patrick Huyghe, Conan the Bacterium, p. 16-19
*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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