Bacterium Deinococcus (Micrococcus) radiodurans is one the most radiation-resistant organisms yet discovered. According to Michael J. Daly, Ph.D., associate professor at the Uniformed Services University of the Health Sciences, the bacterium is “best known for its ability to survive extremely high doses of acute ionizing radiation (10,000 Gy) without cell-killing. For comparison, 5 Gy is lethal to the average human, and 1,000 Gy can sterilize a culture of Escherichia coli. D. radiodurans is capable of growth under chronic radiation (60 Gy/hour) and resistant to other DNA damaging conditions including exposure to desiccation, UV light, and hydrogen peroxide.”
So, what accounts for the ability of this superbug to resist radiation? Dr. Daly and colleagues report the discovery of a biochemical mechanism that protects the bacterium’s proteins from oxidative damage during irradiation.
From the editorial accompanying the original article at PLoS Biology:
Exposing cells to IR generates a range of potentially harmful molecules called reactive oxygen species (ROS). When ROS accumulate faster than cellular scavengers can neutralize them, they cause oxidative stress and can kill cells. Hydroxyl radicals, one of the primary ROS products of irradiated water (the major component of cells), are particularly toxic to DNA, and can generate other ROS, including hydrogen peroxide and superoxide (a simple peroxyl radical).
High intracellular concentrations of manganese ions are known to alleviate oxidative stress in several bacterial species; these ions can interact with different ROS depending on their oxidation state and their binding with different molecules. Daly et al. reasoned manganese might affect ROS generation during irradiation. They first tested manganese’s ability to scavenge hydroxyl and superoxide radicals to determine whether its activity protects DNA or proteins. Whereas hydroxyl radicals target both DNA and proteins, superoxide radicals selectively damage proteins. The researchers irradiated DNA and a DNA-modifying enzyme and found that, although manganese ions failed to protect DNA from hydroxyl radicals generated during irradiation, the ions did prevent enzyme damage and preserved enzyme activity.
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.
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.
Top picture: Image overlay of transmission electron microscopy, light microscopy, and X-ray fluorescence microprobe analyses of D. radiodurans. Average abundance of manganese (blue, green, and pink) and iron (red) are shown within a single D. radiodurans diplococcus.
Editorial: Paradox Resolved? The Strange Case of the Radiation-Resistant Bacteria…
Paper: Protein Oxidation Implicated as the Primary Determinant of Bacterial Radioresistance…
Deinococcus radiodurans – a radiation-resistant bacterium project page…
Press release: One small step for Deinococcus or one giant leap for radiation biology?…
