Image overlay of transmission electron microscopy, light
microscopy, and X-ray fl uorescence microprobe analyses of D.
radiodurans . Average abundance of
manganese (blue, green,
and pink) and iron (red) are shown within a single D. radiodurans
diplococcus.
the effects of IR by protecting
proteins using manganese,
specifi cally 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.
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 benefi t from treatments that deliver purifi ed
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-metalcontaminated
waste sites left over from the Cold War.
More abstracts about the Paradox Resolved? The Strange Case of the Radiation-Resistant Bacteria