Electron micrograph of a cross-section of a D. radiodurans tetracoccus (cluster of four cells).Fifty years ago, scientists experimenting with gamma radiation to sterilize canned foods were surprised to find spoiled meat in cans zapped with what they thought were lethal levels of ionizing radiation (IR). Inside the bulging cans, they discovered a strain of bacteria now called Deinococcus radiodurans. This extremely resilient microbe can endure 100 times the IR levels that kill other bacteria and levels 2,000 times higher than the lethal human dose.
A 2004 study by Michael Daly et al. found that IR-resistant and IR-sensitive cells had significantly different mineral concentrations, lending support to a role of manganese and iron in recovery. The researchers showed that the most resistant cells contained about 300 times more manganese and three times less iron than the most sensitive cells. In a new study investigating the functional consequences of this disparity, Daly et al. show that high cytosolic manganese and low iron concentrations facilitate resistance by protecting proteins, but not DNA, from IR-induced oxidative damage. Their findings offer a novel perspective on the long-cryptic nature of D. radiodurans resistance, shifting the focus of toxicity and resistance away from DNA damage and repair toward a potent form of protein protection.
To understand the nature of manganese protection in cells, the researchers then irradiated IR-sensitive and IR-resistant bacteria and compared their levels of oxidative protein damage. The sensitive cells with the lowest manganese to iron concentration ratios, they found, sustained high levels of protein oxidation; the resistant cells with the highest ratios had no detectable protein oxidation. They showed that proteins purified from D. radiodurans are not inherently oxidation-resistant, and when cells were depleted of manganese, cells were rendered sensitive to IR and protein oxidation. This suggests that the microbe actively offsets the effects of IR by protecting proteins using manganese, specifically with divalent manganese (Mn(II)) ions.
Electron micrograph of a cross-section of a D. radiodurans tetracoccus (cluster of four cells).
Resistant bacteria, the researchers suspected, might use Mn(II) to transform superoxide radicals, which can’t easily cross the cell membrane, into hydrogen peroxide, which can. And that’s what they found: irradiated D. radiodurans and a second resistant bacteria with high manganese concentrations (Lactobacillus plantarum) released hydrogen peroxide (likely as a product of the “redox” reactions that neutralize superoxide radicals), while sensitive and non-irradiated resistant bacteria did not. The researchers went on to show that the resistance of normal D. radiodurans can be controlled externally by inhibiting manganese redox recycling.
In the context of previous studies, these results suggest that D. radiodurans relies not on a highly specialized DNA repair machinery, but on a detoxifying mechanism associated with the microbe’s unusual intracellular environment. Most organisms contain near-millimolar concentrations of iron, which under IR will contribute to the formation of hydroxyl radicals and superoxide radicals. In resistant bacteria, millimolar Mn(II) concentrations appear to protect proteins from oxidative damage by eliminating superoxide and its derivatives. This oxidative protection may in turn shield proteins involved in DNA repair, and subsequently allow them to quickly heal DNA lesions, which in sensitive bacteria turn lethal because their repair proteins are ravaged by radiation.
The Implications:
This new model of radiation toxicity opens up novel avenues for radioprotection in diverse settings. Individuals exposed to chronic or acute doses of radiation could potentially benefit from treatments that deliver purified D. radiodurans Mn complexes into their cells. Similarly, the toxic effects of radiation therapy in cancer patients might be ameliorated by antioxidant drugs based on such a protection paradigm. And given that many bacteria, such as S. oneidensis, with favorable bioremediation functions are extremely sensitive to radiation, the new insight on how D. radiodurans survives radiation might prove useful in efforts to contain the toxic runoff from the immense radioactive- and heavy-metal-contaminated waste sites left over from the Cold War.
These findings are reprinted from PLOS Biology Research Article Written by Liza Gross.
My Take:
I find it extremely significant to know that there are such viable bacteria on earth and this should not be at all new to us since we are already aware of the possibility of some forms of bacteria that can remain dormant and thus be totally capable of enduring space travel and the various and rigorous fluctuations in temperature and environments. Some forms of bacteria can survive in acids so is it even conceivable that life on this planet may have travelled vast distances in space to land on good old mother earth in the form of bacteria and viruses and/or are our bacteria and viruses for that matter leaving mother earth and heading for some distant home in our galaxy carrying their mutations from one planet to another. What we do here on earth, as we always seem to realize down the road, has far reaching implications to others and perhaps to others yet to exist.
