Microbial ‘Superhero’ Yields Significant New Survival Secrets

“If there’s a superhero microbe, it’s D. radiodurans,” says Michael J. Daly from the Uniformed Services University of the Health Sciences in Bethesda, Md, which is why he and his team have made this “endlessly fascinating, but very stubborn and malodorous microbe” their research bacterium of choice; noting that its survival secrets could ultimately be used to protect humans.

In 2007, Daly showed that D. radiodurans accomplishes its astonishing survival feats in an unexpected way — by protecting its proteins, not DNA, with accumulated manganese (Mn²⁺) ions, thus sparing a sufficient number of enzymes critical for genome repair. (See Microbe July 2007 http://www.asm.org/microbe/index.asp?bid=51529)

This year, Daly is discovering exactly how D. radiodurans and other similarly gifted microbes achieve their protein-sparing maneuvers, reporting in the March issue of Nature Reviews Microbiology that “what really counts is not just Mn²⁺ accumulation, but the balance between Mn²⁺ and Fe²⁺ as well as the ability of manganese to form free-radical devouring chemical complexes.” He and his team have demonstrated that microbes with high manganese-to-iron ratios are extremely resistant to gamma radiation-induced protein oxidation while those with low manganese to iron ratios are hypersensitive. Notably, D. radiodurans, which has very efficient systems for Mn²⁺ uptake, typically accumulates 100 times more manganese than radiation-sensitive bacteria.

“Unlike ferrous ions (Fe²⁺),” Daly says, “Mn²⁺ ions are innocuous in aerobic environments with virtually no negative redox consequences; Fe²⁺ but not Mn²⁺ catalyzes the Fenton reaction, one of the most powerful oxidizing reactions known.” The keys to extreme radiation- and desiccation- resistance in Deinococcus are formation of superoxide-scavenging Mn²⁺-phosphate complexes and the accumulation of hydroxyl radical-consuming small organic molecules.

“Importantly,” he adds, “X-ray fluorescence microspectroscopy has just shown that manganese is dispersed throughout D. radiodurans but much of its iron is partitioned between dividing cells, which helps explain how global enzyme protection is accomplished. Because the hydrogen peroxide (H₂O₂) generated during irradiation diffuses widely, manganese and iron portioning serves to minimize the Fenton reaction.”

By comparison, iron-rich and manganese-poor bacteria suffer a torrent of reactive oxygen species (ROS) during irradiation which inactivate most enzymes. “Unless an irradiated cell can protect its enzymes from oxidation, even the most minor DNA damage will kill it,” emphasizes Daly.

Promisingly, diploid yeast cells exposed to the most extreme genomic insults caused by gamma-radiation can repair and rebuild their genomes, suggesting that complex eukaryotes have the potential to survive high doses of ionizing radiation. To this end, Daly is developing a series of “Deinococcus-inspired” radioprotectants. By combining Mn²⁺ with ligands such as phosphate and other small molecules, Daly hopes to chemically shield mammalian repair enzymes from radiation injury. “The right mix,” he says, “when delivered into human cells could spontaneously form intracellular complexes that scavenge superoxide and related ROS.”

On Daly’s short list of potential applications are: “making radiation-therapy more tolerable for cancer patients; protecting astronauts from radiation during long-duration space travel; cleaning up the slumgullion of radioactive waste left over from the Cold War; and developing ways to slow down the aging process.”

“I’m excited,” says Daly, “that in the last few years, this research has moved from the realm of science fiction to plausible reality”.

Daly’s work is being closely watched by other scientists, including cell biologist Colin Dingwall at Kings College London who has shown that BACE1, or beta-secretase, a principal component of senile plaques, is intimately linked to copper —another ROS-generating metal— in the brains of Alzheimer’s patients. Dingwall says that “substituting Mn²⁺ for Cu⁺ to prevent redox chemistry is an interesting idea and if it works, he speculates that small molecules such as peptides might be used as delivery agents.”

“Daly’s convincing demonstration that simple manganese complexes protect proteins from oxidative damage in vivo makes me wonder if manganese is acting similarly in more complex organisms,” says biochemist Joan S. Valentine from the University of California Los Angeles, adding that “perhaps the antioxidant effects of manganese supplementation that we’ve been attributing to increases in manganese superoxide dismutase enzymes might really be due to simple manganese complexes.”

Thinking about how D. radiodurans evolved its manganese-based resistance to radiation, microbial ecologist Tom Curtis from the University of Newcastle upon Tyne in England expects “it’s likely adapted to sunlight, at least in the Antarctic, because they’re already dried out when they land here.” Curtis who investigates the “preposterously high levels of microbial diversity” in that part of the world explains that “photons of sunlight are absorbed into the bacterial cell, probably by cytochromes, reacting with oxygen to form oxygen radicals.” Daly’s research, he says, reveals how Deinococci stays alive despite the assaults.

Rodney L. Levine from NIH’s National Heart, Lung, and Blood Institute agrees with Curtis adding that “perhaps it’s sunlight, perhaps it’s desiccation, organisms which evolve or induce a resistance to one stress are often resistant to multiple other stresses.” But, he says, “as Daly has pointed out, we live in a DNA-centric world which holds that cells die because of genome injury and this is not entirely correct. Deinococcus’ DNA is as diced and sliced by irradiation as that of E. coli, but Deinococcus survives when Escherichia dies. Daly’s experimental data show why this happens; it’s all about the proteins.”