|A tetrad of D. radiodurans|
Brooks & Murray, 1981
Deinococcus radiodurans is an extremophilic bacterium, one of the most radioresistant organisms known. It can survive cold, dehydration, vacuum, and acid, and is therefore known as a polyextremophile and has been listed as the world's toughest bacterium in The Guinness Book Of World Records.
The name Deinococcus radiodurans derives from the Greek deino and kokkos meaning "terrible berry" and the Latin radius and durare meaning "radiation surviving". The species was formerly called Micrococcus radiodurans. As a consequence of its hardiness, it has been nicknamed "Conan the Bacterium", a play on "Conan the Barbarian".
Initially it was placed in the genus Micrococcus. After evaluation of ribosomal RNA sequences and other evidence, it was placed in its own genus Deinococcus, which is closely related to the genus Thermus of heat-resistant bacteria; the group consisting of the two is accordingly known as Deinococcus-Thermus.
Deinococcus is the only genus in the order Deinococcales. D. radiodurans is the type species of this genus, and the best studied member. All known members of the genus are radioresistant: D. proteolyticus, D. radiopugnans, D. radiophilus, D. grandis, D. indicus, D. frigens, D. saxicola, D. marmoris, D. deserti, D. geothermalis and D. murrayi; the latter two are also thermophilic.
D. radiodurans was discovered in 1956 by Arthur W. Anderson at the Oregon Agricultural Experiment Station in Corvallis, Oregon. Experiments were being performed to determine if canned food could be sterilized using high doses of gamma radiation. A tin of meat was exposed to a dose of radiation that was thought to kill all known forms of life, but the meat subsequently spoiled, and D. radiodurans was isolated.
Deinococcus radiodurans has a unique quality in which it can repair DNA both single and double stranded. When a mutation is apparent to the cell it brings it into a compartmental ring like structure where the DNA is repaired and then is able to fuse the nucleoids from the outside of the compartment with the damaged DNA.
D. radiodurans is a rather large spherical bacterium, with a diameter of 1.5 to 3.5 µm. Four cells normally stick together, forming a tetrad. The bacteria are easily cultured and do not appear to cause disease. Colonies are smooth, convex, and pink to red in color. The cells stain gram positive, although its cell envelope is unusual and is reminiscent of the cell walls of gram negative bacteria. 
D. radiodurans does not form endospores and is nonmotile. It is an obligate aerobic chemoorganoheterotroph, i.e. it uses oxygen to derive energy from organic compounds in its environment. It is often found in habitats rich in organic materials, such as soil, feces, meat, or sewage, but has also been isolated from dried foods, room dust, medical instruments and textiles.
Its genome consists of two circular chromosomes, one 2.65 million base pairs long and the other 412,000 base pairs long, as well as a megaplasmid of 177,000 base pairs and a plasmid of 46,000 base pairs. It has about 3,195 genes. In its stationary phase each bacterial cell contains four copies of this genome; when rapidly multiplying, each bacterium contains 8-10 copies of the genome.
D. radiodurans is capable of withstanding an instantaneous dose of up to 5,000 Gy of ionizing radiation with no loss of viability, and an instantaneous dose of up to 15,000 Gy with 37% viability. A dose of 5,000 Gy is estimated to introduce several hundred complete breaks into the organism's DNA. For comparison, a chest X-ray or Apollo mission involves about 1 milligray, 10 Gy can kill a human, 60 Gy will kill E. coli, and over 4000 will kill the radiation-resistant tardigrade.
Several bacteria of comparable radioresistance are now known, including some species of the genus Chroococcidiopsis (phylum cyanobacteria) and some species of Rubrobacter (phylum actinobacteria); among the archaea, the species Thermococcus gammatolerans shows comparable radioresistance. Deinocuccus radiodurans also has a unique ability to repair damaged DNA. It isolates the damaged segments in a controlled area and repairs it. This bacteria can also repair many small fragments from and entire chromosome.
Deinococcus accomplishes its resistance to radiation by having multiple copies of its genome and rapid DNA repair mechanisms. It usually repairs breaks in its chromosomes within 12–24 hours through a 2-step process. First, D. radiodurans reconnects some chromosome fragments through a process called single-strand annealing. In the second step, a protein mends double-strand breaks through homologous recombination. This process does not introduce any more mutations than a normal round of replication would.
A persistent question regarding D. radiodurans is how such a high degree of radioresistance could evolve. Natural background radiation levels are very low—in most places, on the order of 0.4 mGy per year, and the highest known background radiation, near Ramsar, Iran is only 260 mGy per year. With naturally-occurring background radiation levels so low, organisms evolving mechanisms specifically to ward off the effects of high radiation are unlikely.
Valerie Mattimore and John R. Battista of Louisiana State University have suggested that the radioresistance of D. radiodurans is simply a side-effect of a mechanism for dealing with prolonged cellular desiccation (dryness). To support this hypothesis, they performed an experiment in which they demonstrated that mutant strains of D. radiodurans which are highly susceptible to damage from ionizing radiation are also highly susceptible to damage from prolonged desiccation, while the wild type strain is resistant to both. In addition to DNA repair, D. radiodurans use LEA (Late Embryogenesis Abundant) protein expression to protect against desiccation.
A team of Croatian and French researchers leaded by Miroslav Radman have bombarded D. radiodurans to study the mechanism of DNA repair. At least two copies of the genome, with random DNA breaks, can form DNA fragments through annealing. Partially overlapping fragments are then used for synthesis of homologous regions through a moving D-loop that can continue extension until they find complementary partner strands. In the final step there is crossover by means of RecA-dependent homologous recombination.
Michael Daly has suggested that the bacterium uses manganese as an antioxidant to protect itself against radiation damage. In 2007 his team showed that high intracellular levels of manganese(II) in D. radiodurans protect proteins from being oxidized by radiation, and proposed the idea that "protein, rather than DNA, is the principal target of the biological action of [ionizing radiation] in sensitive bacteria, and extreme resistance in Mn-accumulating bacteria is based on protein protection".
A team of Russian and American scientists proposed that the radioresistance of D. radiodurans had a Martian origin. Evolution of the microorganism could have taken place on the Martian surface until it was delivered to Earth on a meteorite. However, apart from its resistance to radiation, Deinococcus is genetically and biochemically very similar to other terrestrial life forms, arguing against an extraterrestrial origin.
In 2009 it was reported that nitric oxide plays an important role in the bacteria's recovery from radiation exposure: the gas is required for division and proliferation after DNA damage has been repaired. A gene was described that increases nitric oxide production after UV radiation, and in the absence of this gene the bacteria were still able to repair DNA damage but would not grow.
Deinococcus has been genetically engineered for use in bioremediation to consume and digest solvents and heavy metals, even in a highly radioactive site. For example, the bacterial mercuric reductase gene has been cloned from Escherichia coli into Deinococcus to detoxify the ionic mercury residue frequently found in radioactive waste generated from nuclear weapons manufacture. Those researchers developed a strain of Deinococcus that could detoxify both mercury and toluene in mixed radioactive wastes.
The Craig Venter Institute has used a system derived from the rapid DNA repair mechanisms of D. radiodurans to assemble synthetic DNA fragments into chromosomes, with the ultimate goal of producing a synthetic organism they call Mycoplasma laboratorium.
In 2003, U.S. scientists demonstrated that D. radiodurans could be used as a means of information storage that might survive a nuclear catastrophe. They translated the song It's a Small World into a series of DNA segments 150 base pairs long, inserted these into the bacteria, and were able to retrieve them without errors 100 bacterial generations later.