Radiation-resistant bacteria encompass eight species of bacteria in a genus known as Deinococcus. The prototype species is Deinococcus radiodurans. This and the other species are capable of not only survival but of growth in the presence of radiation that is lethal to all other known forms of life.
Radiation is measured in units called rads. An instantaneous dose of 500 to 1000 rads of gamma radiation is lethal to a human. However, Deinococcus radiodurans is unaffected by exposure to up to 3 million rads of gamma radiation. Indeed, the bacterium, whose name translates to "strange berry that withstands radiation," holds a place in The Guinness Book of World Records as "the world's toughest bacterium."
The bacterium was first isolated in the 1950s from tins of meat that had spoiled in spite of being irradiated with a dose that was thought to be sterilizing. The classification of the bacterium as Deinococcus radiodurans, and the isolation, characterization, and designation of the other species has been almost exclusively due to Robert Murray and his colleagues at the University of Western Ontario. The various species of Deinococcus have been isolated from a variety of locations as disperse as elephant feces, fish, fowl, and Antarctic rocks.
The reason for the development of such radiation resistance is still speculative. But, the current consensus is that it enabled the ancient form of the bacterium to survive in regions where available water was scarce. Other organisms developed different survival strategies, one example being the ability to form the metabolically dormant spore.
Deinococcus is an ancient bacteria, believed to be some two billion years old. They may have evolved at a time when Earth was bathed in more energetic forms of radiation than now, due to a different and less screening atmosphere. One theory even suggests that the bacteria originated on another world and were brought to Earth via a meteorite.
The extraterrestrial theory is likely fanciful, however, because the bacteria are not heat resistant. Exposure to temperatures as low as 113ºF (45ºC) can be lethal to the microorganism.
There are two known reasons for the radiation resistance of species of Deinococcus. Firstly, the structure of the two membranes that surround the Gram-negative bacterium contributes, albeit in a minor way. By far the major reason for the radiation resistance is the bacterium's ability to rapidly and correctly repair the extensive damage caused to its genetic material by radiation.
The high energy of radioactive waves literally cut apart the double stranded molecule of deoxyribonucleic acid (DNA ). These cuts occur in many places, effectively shattering the genome into many, very small fragments. Deinococcus is able to quickly reassemble the fragments in their correct order and then slice them back together. In contrast, bacteria such asEscherichia coli can only tolerate one or several cuts to the DNA before the radiation damage is either lethal or causes the formation of drastic mutations .
The molecular nature of this repair ability is not yet clear. However, the completion of the sequencing of the genome of Deinococcus radiodurans in late 1999 should provide the raw material to pursue this question. The genome is unique among bacteria, being comprised of four to ten pieces of DNA and a large piece of extrachromosomal DNA that is part of a structure called a plasmid. The genome of other bacteria typically consists of a single circle of DNA (although plasmid DNA can also be present). Within the chromosome-like regions of Deinococcus there are many repeated stretches of DNA. In an analogy to a computer, the bacterium has designed many backup copies of its information. If some back up copies are impaired, the information can be recovered from the other DNA.
This DNA repair ability has made the genus the subject of intense scrutiny by molecular biologists interested in the process of DNA manufacture and repair. Furthermore, the radiation resistance of Deinococcus has made the bacteria an attractive microorganism for the remediation of radioactive waste. While this use is not currently feasible at the scale that would be required to clean up nuclear contamination, small-scale tests have proved encouraging. The bacteria still need to be engineered to cope with the myriad of organic contaminants and heavy metals that are also typically part of nuclear waste sites.
See also Bioremediation; Extremophiles