Conan the Bacterium

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Conan the Bacterium

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Deinococcus radiodurans ("strange berry that withstands radiation",
formerly called Micrococcus radiodurans) is an extremophilic
bacterium, and is the most radioresistant organism known. While a dose
of 10 Gy is sufficient to kill a human, and a dose of 60 Gy is
sufficient to kill all cells in a culture of E. coli, D. radiodurans
is capable of withstanding an instantaneous dose of up to 5,000 Gy
with no loss of viability, and an instantaneous dose of up to 15,000
Gy with 37% viability. It can survive heat, cold, dehydration, vacuum,
and acid, and because of its resistance to more than one extreme
condition, D. radiodurans is known as a polyextremophile. It has also
been listed as the world's toughest bacterium in "The Guiness Book Of
World Records" because of its extraordinary resistance to several
extreme conditions. It has been classified as a Gram-positive
bacterium.

The term Deinobacter has been replaced by Deinococcus based on
evaluation of ribosomal RNA sequences. Several other species within
the genus have been described, and they are related to heat-resistant
bacteria such as Thermus; the group is accordingly known as
Deinococcus-Thermus. HistoryD. radiodurans was discovered in 1956 by
A.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. D.
radiodurans was isolated from the meat. Radioactivity resistance
mechanismsDeinococcus 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. As a consequence of its hardiness it has
been nicknamed "Conan the Bacterium" (after Conan the Barbarian).

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 Guarapari,
Brazil is only 175 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 [1]. In
addition to DNA repair, D. radiodurans use LEA (Late Embryogenesis
Abundant) protein[2] expression to protect against desiccation[3].

Michael Daly of the Uniformed Services University of the Health
Sciences suggests that the bacterium uses manganese to protect itself
against radiation damage[4].

Scanning electron microscopy analysis has shown that DNA in D.
radiodurans is organized into tightly packed toroids, which may
facilitate DNA repair[5].

A team of Croatian and French researchers 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 [6].

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. [7]. It has been
hypothesised that such objects might have attained sufficient kinetic
energy to achieve escape velocity as a result of a major meteorite
impact on the Martian surface (this being part of a hypothesised
process known as panspermia). The heating effects of that impact and
of entry into the Earth's atmosphere would have had to have been
insufficient to sterilise such an object. However, apart from its
resistance to radiation, Deinococcus is genetically and biochemically
quite similar to other terrestrial life forms, arguing against an
extraterrestrial origin. ApplicationsUsing genetic engineering
Deinococcus has been used for bioremediation to consume and digest
solvents and heavy metals, even in a highly radioactive site. The
bacterial mercuric reductase gene has been cloned from Escherichia
coli into Deinococcus to detoxify the ionic mercury frequently found
in radioactive waste generated from nuclear weapons manufacture[8].
Those researchers developed a strain of Deinococcus that could
detoxify both mercury and toluene in mixed radioactive wastes.

Some have speculated that mechanisms of DNA repair used by D.
radiodurans could be incorporated into the genome of higher species as
a means of rejuvenation[6].

Some scientists have suggested that D. radiodurans could be
genetically manipulated to produce various medicines.

The Craig Venter Institute has used a system derived from the rapid
DNA repair mechanisms of D. radiodurans to assemble synthetic genes
into chromosomes to make synthetic organisms [10].

Trivia
The computer game Anarchy Online features an item called Deinococcus
radiodurans which is used to make a "stim" (potion) that reduces
damage from radioactive sources.