Scientists looking for a companion bacterium to take on a trip to Mars would do well to choose Deinococcus radiodurans. This quintessential polyextremophile is virtually impervious to desiccation and easily survives high levels of chemical, oxidative, ultraviolet and ionizing radiation-induced DNA damage - traits that would undoubtedly work as well in harsh, alien environments as they do in the extreme habitats on Earth.
No wonder then that this exceptional little microbe has been the polyextremophile-of-choice for NASA’s Martian studies since the late 1990s; its jobs include helping scientists find sites with the greatest probability of hosting life and pointing to extraterrestrial habitats that can support human survival.
Here on earth, “Desiccated Deinococci can endure for years and if blown by winds through the atmosphere will survive and land worldwide,” according to Deinococcus expert Michael J Daly PhD, from the Uniformed Services University of the Health Sciences in Bethesda, Md. Some, he says, are found encased in ice and others are entombed in dry desert soils. “Understanding why D. radiodurans is so resilient gives us ways to protect people from atomic radiation, clean up radioactive waste sites and live longer,” Daly stresses.
Radiation “Diced and Sliced” DNA Fixed by Protected Proteins
In 2007, Daly showed that D. radiodurans accomplish their astonishing survival feats in an unexpected way –by protecting their proteins with manganese ions. This spares a sufficient number of DNA repair enzymes to reassemble its genome, which is as "diced and sliced" by irradiation as the DNA of other bacteria. But whereas D. radiodurans survive, microbes like Escherichia coli die and it's all about protecting the proteins, "which is why Daly's experiments are so important in this DNA-centric scientific world," notes Rodney L. Levine MD, PhD from NIH’s National Heart, Lung, and Blood Institute in Bethesda, Md.
Manganese Prevents Iron-Induced Oxidative Stress
This past year Daly discovered exactly how D. radiodurans and other similarly gifted microbes achieve their protein-sparing maneuvers; “what really counts isn’t just manganese accumulation,” he says, “but the small molecules which are co-accumulated by the cells - phosphate, small peptides, and other common metabolites --together with the cells' low intracellular concentrations of iron."
He and his team had already shown that microbes with high manganese to iron ratios are extremely resistant to ionizing radiation and those with low manganese to iron ratios are radiation hypersensitive. This was documented by X-ray fluorescence microspectroscopy which also showed that while manganese atoms are dispersed throughout D. radiodurans, poised to mop up dangerous free oxygen radicals, dangerous metals such as iron and copper are jettisoned and sequestered in spaces between dividing cells.
According to Daly, manganese trumps iron in two ways during a radiation assault: 1. by preventing rather than catalyzing the release of reactive oxygen species (ROS) from water, the most abundant chemical in living cells; and 2. by forming potent free radical-scavenging manganese complexes to inactivate the many ROS which are formed. ROS are known to cause a great deal of cellular harm including rapid enzyme inactivation and double-stranded DNA breaks, the most lethal form of damage DNA can sustain.
Stress-Resistant Microbes also Call the Human Gut Home
Surprising to some, there are lots of radiation and desiccation-resistant microbes on earth and they like to swap genes --"bacteria and archaea routinely pass their genes around and these are genes worth having because they control the metabolic processes responsible for promoting the formation of antioxidant manganese complexes," stresses Daly. Some mighty-microbes, even Deinococcus, end up in human intestinal tracts where they synthesize and accumulate potent manganese antioxidant complexes which are eventually released into the environment. Indeed, a spell in the nutrient-rich gut of man or other animals seems mandatory for rapid growth of most earthly Deinococcus.
Other harmless radiation-resistant microbes such as lactobacilli, the “probiotic” bacteria in yogurts, also accumulate very high manganese concentrations, which could explain their gastrointestinal benefits. “All of these bacteria are little antioxidant factories,” Daly says and Levine adds that this finding “may provide a rational basis for intervention in processes which generate oxidative damage, including aging.”
Protecting People the Deinococcus Way
Thus, a microbe with the exceptional endurance traits enabling it to repair and recover from accumulated genetic damage; one that “could likely survive extended periods on the hostile surface of Mars,” says Daly, points the way to better health in humans here on earth. Daly, for example, is developing “Deinococcus-inspired” manganese-based radioprotectants and agents which slow down down the aging process. “The right mix,” he thinks, “when delivered into human cells could scavenge a variety of common ROS generated by radiation and metabolism." Indeed, recent studies demonstrate that human cells exposed to very high doses of gamma-rays in liquid culture are rescued by Deinococcus-derived manganese complexes.
In the meanwhile, it seems wise to nourish the radiation-resistant bacteria we all have with manganese-rich foods such as lettuce, mustard greens, Swiss chard, pineapples, garlic, blackstrap molasses, brown rice and green tea. As Daly advises, "eat up your green veggies and stay off the [iron laden] red meats.".
Update on Deinococcus radioduran research in this December's Microbe magazine.
Research details can be found in:
Daly MJ. A new perspective on radiation resistance based on Deinococcus radiodurans. Nature Reviews Microbiology 27 January 2009; doi:10.1038/nrmicro2073.
Fredrickson JK, Li SW, Gaidamakova EK et al. Protein oxidation: key to bacterial desiccation resistance? The ISME Journal 2008 (2), pp. 393-403.
Daly MJ, Gaidamakova EK, Matrosova VY, et al. Protein oxidation implicated as the primary determinant of bacterial radioresistance. PLoS Biol 2007; Apr; 5(4)e92.
Daly MJ, Gaidamakova EK, Matrosova VY et al Accumulation of Mn(II) in Deinococcus radiodurans Facilitates Gamma-Radiation Resistance.Science 5 November 2004; 306: pp. 1028.
Richmond RC, Sridhar R, Zhou Y, Daly MJ. Physico-chemical survival pattern for the radiophile D. radiodurans:A polyextremophile model for life on Mars. Part of the SPIE conference on Instruments, Methods, and Missions for Astrobiology ** Denver, Colorado; July 1999· July 1999 SPIE Vol. 3755· 0277-786X/99.
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