Deinococcus radiodurans, a radioresistant extremophile. 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.
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. As a consequence of its hardiness, it has been nicknamed “Conan the Bacterium”.
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 is capable of withstanding an acute dose of 5,000 Gy of ionizing radiation with almost no loss of viability, and an acute dose of 15,000 Gy with 37% viability. A dose of 5,000 Gy is estimated to introduce several hundred double strand breaks (DSBs) into the organism’s DNA (~0.005 DSB/Gy/Mbp (haploid genome)). For comparison, a chest X-ray or Apollo mission involves about 1 mGy, 5 Gy can kill a human, 200-800 Gy will kill E. coli, and over 4,000 Gy will kill the radiation-resistant tardigrade.
Deinococcus radiodurans, episode 2: How'd it get so tough?
Well, it was probably just luck. We now know that D. radiodurans shares a recent common ancestor with T. aquaticus (in fact, the two impressive organisms are grouped into one phylum, Deinococcus-Thermus). The ancestor likely had some genetic predisposition to tolerating stressful environments that it passed on to these and other species. From there, D. radiodurans got yet more stress response genes from other bacteria through a process called horizontal gene transfer. So that explains its resistance to some common conditions that many extremophiles are able to survive, like cold and acidity. But what about all that radiation, which doesn’t occur anywhere in nature? That seems to be a happy accident from the evolution of resistance to conditions that we do find in nature: extreme dryness, or desiccation. Desiccation causes DNA to break into small pieces in the same way radiation does, so the ability to repair that damage would help an organism survive both kinds of extreme conditions. That organism might then earn an awesome nickname, like “Conan the Bacterium”: