Deinococcus radiodurans is a bacterium that has earned the moniker “Conan the Bacterium” for its unparalleled ability to withstand environmental extremes. This extremophile is the most radiation-resistant organism currently known on Earth. Its remarkable resilience allows it to survive conditions that would instantly kill nearly all other forms of life. The organism’s name translates roughly to “terrible berry that endures radiation.”
Discovery and Defining Characteristics
Deinococcus radiodurans was first discovered in 1956 at the Oregon Agricultural Experiment Station in Corvallis. This occurred during efforts to sterilize canned meat using high-dose gamma radiation. Scientists were astonished when the meat spoiled despite receiving a radiation dose thought sufficient to kill every known microbe, revealing the presence of this stubborn bacterium. The bacterium is a non-motile, red-pigmented organism that typically exists in clusters of four cells, known as a tetrad.
Its resistance extends beyond radiation, classifying it as a polyextremophile that tolerates a wide range of harsh conditions. While ubiquitous, it is not a strong competitor in benign environments, suggesting its unique survival mechanisms allow it to thrive where other organisms cannot. The cell is characterized by an unusually thick cell wall composed of up to six layers, which contributes to its hardiness.
Mechanisms of Extreme Endurance
The bacterium’s endurance allows it to survive acute ionizing radiation doses of up to 15,000 Gray (Gy). This amount is 3,000 times higher than the lethal dose for humans, which is only about 5 Gy. This tolerance is not limited to gamma radiation; it also demonstrates resistance to ultraviolet (UV) radiation, desiccation, and vacuum. The ability to survive extreme dryness is noteworthy because desiccation causes DNA fragmentation comparable to that inflicted by radiation.
A primary cellular strategy involves minimizing damage to the cell’s proteins, or proteome, which are necessary for cellular function and repair. Unlike many organisms where cell death correlates with protein damage, D. radiodurans maintains a protected and functional set of proteins, even when its DNA is shattered. This protection is facilitated by a high concentration of manganese ions and a low concentration of iron within the cell. Manganese is theorized to act as an antioxidant, scavenging the reactive oxygen species (ROS) produced by radiation that would otherwise destroy proteins and lipids.
The Unique DNA Repair System
The bacterium’s resilience stems from its capacity to repair its severely damaged genome. A dose of radiation that D. radiodurans can survive can fragment its chromosome into hundreds of pieces. The bacterium can then accurately reassemble these fragments into a functional genome within a few hours.
This efficiency stems partly from its polyploidy, meaning a single cell contains between four and ten copies of its entire genome, providing multiple templates for repair. The DNA is also organized into a condensed, ring-like structure called a toroid. This structure prevents the broken fragments from diffusing away after damage, allowing for rapid and organized reassembly.
The primary process for repairing these double-strand breaks is the Extended Synthesis-Dependent Strand Annealing (ESDSA) mechanism. This process involves the synthesis of new DNA using the multiple genome copies as guides to fill in the gaps created by the fragmentation. Proteins like RecA, which are common in many bacteria, play a role in this recombination repair. The D. radiodurans version of the pathway is highly amplified and distinct. The combination of multiple genome copies, toroidal DNA packaging, and the specialized ESDSA pathway allows for the reconstruction of an intact genome without mutations.
Scientific Utility and Bioremediation Potential
The hardiness of D. radiodurans makes it valuable for scientific research, particularly in the field of bioremediation. Scientists are leveraging its resistance to engineer strains capable of cleaning up toxic waste in highly contaminated environments. These environments, such as nuclear waste sites, often contain high-level radiation mixed with toxic heavy metals or organic solvents.
Engineered strains of the bacterium can be modified to express genes that allow them to consume or neutralize these hazardous substances. For example, a strain has been developed to detoxify both ionic mercury and toluene while surviving chronic radiation exposure. The wild-type bacterium naturally reduces toxic hexavalent chromium (Cr(VI)) to the less harmful trivalent chromium (Cr(III)), highlighting its potential for environmental cleanup. The bacterium is also being explored in synthetic biology as a model for understanding and enhancing DNA stability in other organisms.