Meet Conan the Bacterium

December 13, 1999

Humble microbe could become "The Accidental (Space) Tourist"

Dec. 14, 1999: Like a muscle-bound movie hero, it withstands attacks from acid baths, high and low temperatures, and even radiation doses. Then, in a science fiction sequel, it dispenses lifesaving medications and reshapes a planet for new settlers. And in true Hollywood fashion, the star of this epic had humble beginnings, living in cow patties and elephant dung, and coming to the attention of scientists when it refused to die in food sterilization tests. You need a microscope to see this miniature future hero listed as Deinococcus radiodurans and known to its fans as Conan the Bacterium.

"Deinococcus radiodurans beats most of the constraints for survival of life on Mars - radiation, cold, vacuum, dormancy, oxidative damage, and other factors," said Dr. Robert Richmond, a research biologist at NASA's Marshall Space Flight Center. With other scientists, he is investigating the possible utility of extremophiles to serve human exploration to inhospitable locations.

Humble origins

Richmond and his colleagues see D. radiodurans as playing the part of possible Martian microbes in simulations to help direct the search for life on Mars. Next, it could be genetically altered to produce medicines for astronauts in the short-term, rather than hauling an entire pharmacy along on the trip, and restructuring Mars for human habitation in the long-term. With R. Sridhar of Howard University Medical Center in Washington, D.C. and Dr. Michael J. Daly of the Uniformed Services University of the Health Services in Bethesda, Md., Richmond presented a paper at the 1999 SPIE Conference in Denver on the "Physico-Chemical Survival Pattern for the Radiophile D. radiodurans: A Polyextremophile Model for Life on Mars."

Daly and his co-workers, in a recent article in Science magazine, announced that they had completed sequencing the genome of D. radiodurans. This opens the way for exploitation of its ability to integrate external genes selected to express products useful to explorers on Mars or other such places. "Radiodurans' beginnings are thought to be from early Earth," Richmond said, and paralleled a time when the environment may have also approximated that existing on Mars for a few hundred million years. Given the presumed sharing of debris generated from meteorite impacts amongst the early planets, origins of D. radiodurans might even be accidentally common between Mars and Earth. "By nature, it is selected to survive radiation damage very well," D. radiodurans can withstand without loss of viability a dosage that is 3,000 times greater than what would kill a human. "The fact that you can genetically engineer these things is the key to the utility of this bug."

It's heady stuff for a primitive organism.

But D. radiodurans has a feature that is considered all-important in aerospace: redundancy. Its genetic code repeats itself many times so that damage in one area can be recognized and quickly repaired. Coupled with its range of other survival characteristics, D. radiodurans has been dubbed a polyextremophile by Richmond, Sridhar, and Daly.

Extremophiles have been known to scientists for decades but often were regarded a laboratory oddity. The discovery of what appears to be nanobacteria (or nanobes, smaller than microbes) in a meteorite from Mars (Alan Hills 84001, or ALH84001) catapulted extremophiles into the spotlight as a model for possible lifeforms on Mars.

The debate over whether the ALH84001 forms ever were nanobes (or just non-living imitations) led to recent discoveries of probable nanobes living in such odd places as human kidney stones and in limestone 4 kilometers under the surface of the Earth.

A common link?

"We have a new door opening on the possibilities of lifeforms," Richmond said, "not just new species but whole new life forms that could connect to the origins of life on Earth and could be a common link to the possible beginnings of life on Mars."

Most extremophiles have optimized themselves for one or two extreme conditions and settled into wonderful ecological niches like the hot springs of Yosemite. Radiodurans has been dubbed a polyextremophile because it can endure many extremes, including the most dangerous space hazard, radiation.

"Radiation-induced DNA damage is an oxidizing type of damage," Richmond said. It happens when radiation energizes an atom enough to break a chemical bond and then act like an atom of oxygen and bind with another atom. Such free radicals have been implicated in a range of cancers and genetic mutations.

D. radiodurans, though, is hypothesized by Daly to resist such damage by virtue of repair specialized to utilize its redundant strands of DNA. This also means that it should resist damage from the chemistry of Mars, which chemical experiments done by the labs aboard the two Viking landers indicate may be highly oxidative.

D. radiodurans was discovered in the 1950s. Scientists experimenting with radiation to kill bacteria and preserve food for long periods found that something kept growing back after treatment.

It remained a laboratory oddity for several years until the arrival of genetic engineering, the science of altering an organism's basic biological code, sometimes by splicing into it portions of another organism's code. Daly's group is inserting specialized genes to help in eliminating dangerous chemicals from waste sites. An established example of the value of such genetic engineering is found with E. coli, the bacteria found in the human gut, that has been engineered to produce large quantities of human insulin, which once had to be refined from human cadavers.

"Daly has been active in developing D. radiodurans as a special model for bioremediation to clean radioactive supersites left over from the Cold War," Richmond explained. Some of those sites contain radioactive materials that are not easily removed by other microbes. While some other bacteria are being genetically engineered to thrive in toxic conditions while converting hazardous waste into reusable effluent, none can resist radiation the way D. radiodurans can.

Already, Daly and his colleagues have devised D. radiodurans variants that can clean up mercury, a deadly heavy metal, and toluene, a dangerous solvent. This work was sponsored by the U.S. Department of Energy. The capability to insert genes also makes D. radiodurans a candidate for Mars pharmacists and to become "the plow that broke the plains" on Mars.

But first, it may help search for life on Mars as a stand-in for Martian microbes in simulated Mars environments.

The changing face of Mars

Mars has gone through radical changes in our perception as a haven for life. After Sir Percival Lowell and a number of science fiction stories popularized Mars as a dying planet, U.S. space probes in the 1960s and 1970s rewrote the book to show Mars as long dead, perhaps never alive.

Then came the discoveries hidden inside ALH84001. Soon thereafter, images and data from the Mars Global Surveyor, Mars Pathfinder, and Sojourner Rover spacecraft showed Mars indeed has significant quantities of water, and once had running water.

While Mars has become more tantalizing, it is far from Eden. So the question is, if life was there, or is there, what are the best places to find it?

Spacecraft surveying the planet to determine where water might survive beneath the surface, or where it once may have existed, are addressing this. Even within those regions, you have to figure out which spots are best since a lander will have limited time and resources compared to the open wilds of Mars. One approach is to culture D. radiodurans in Mars simulations on Earth.

"We are restricted in the search for life right now to Earth-based microbes," Richmond explained. "We have to ask, What are the restraints on life that those microbes will have to surmount in order to plausibly exist on other planets?" Extremophile habitats on Earth cover a range of conditions: temperatures near boiling or below freezing; a nearly total lack of water, or water that ranges from alkaline to acidic or salty; non-carbon foods; and a lack of oxygen. One of the tricks that less durable lifeforms use to survive such tough times is to hibernate as spores. Such was the case with Streptococcus mitus discovered inside a TV camera recovered by the Apollo 12 crew from the Surveyor 3 spacecraft on the Moon. To everyone's amazement, the bacteria were viable and quickly revived in a culture on Earth. But that was after just a three-year stay.

"The restraints become temporal, too," Richmond explained. "Dormancy has to carry on for thousands or millions of years" if a life form is to last until conditions on Mars become hospitable for growth, somewhat like the floral seeds waiting in the desert for the rare fall of rain.

And that's where radiation resistance comes in handy. While radiation issues are usually associated with nuclear power or exposure to the space environment, it is not commonly recognized as being inescapable. We are exposed through our entire lives to potassium-40, radon, carbon-14 and other radioactive sources. Living in the mountains or flying also increases exposure slightly.

Surviving a long winter's nap

But the total dosage from these is small during our lifespans, so the impact normally is insignificant. However, for an organism in hibernation for a million years or so, the cumulative exposure can be like sitting inside a reactor for several minutes.

That's why crawling under a rock to escape solar ultraviolet light on Mars is not a perfect strategy. The rock itself emits trace quantities of radiation over time.

"Within responsible imagination, no long-dormant lifeform can be expected on the surface of Mars due to combined build up of damage over time caused by both incoming space radiation plus the background radiation," Richmond said. The best hope is that life got started some billions of years ago when conditions were more hospitable, and that a few microbes adapted to extreme conditions or learned how to hibernate below the surface.

"But if they wake up too late, they run into the ultimate restriction, too much radiation damage that has accumulated if it's not repaired," Richmond said. "At that point, the population is dead."

So even if something like D. radiodurans evolved on early Mars, it's possible that winter has lasted too long for any survivors to reawaken in the artificial spring of a petri dish.

Even so, D. radiodurans may yet travel to Mars as a Pharmacist's Mate First Class.

"Because of genetic engineering, you might do a lot with this bug to enhance the survivability of man in extraterrestrial environments," Richmond said. Altering the human genome to take on survival characteristics like D. radiodurans is far too complex a task (the human genome hasn't been completely sequenced, nor all of its 100,000+ genes decoded). But D. radiodurans could be altered to serve man.

"The interesting things about drugs we use is that about two-thirds are natural products or derived from natural products," Richmond said. "Anything that is a natural product ultimately comes down to a gene and can be genetically managed, in theory."

Living off the land - after you reshape it

Richmond, Sridhar, and Daly suggest that D. radiodurans can be genetically manipulated to produce various drugs that humans might need while exploring Mars, then put on ice during the mission. If someone became ill, treatment would start with drugs in from a small supply kept on hand, while the appropriate bugs were awakened to produce a regular supply. (This need was presaged this summer by the need to airdrop tamoxifen, a breast cancer chemotherapy agent, at the South Pole for a medical doctor who had diagnosed herself with breast cancer.) With such an approach, the issues of shelf life for drugs could also be circumvented. This would also reduce the weight that a spaceship would have to haul to Mars and back.

Radiodurans next might be drafted as a Seabee (Navy Construction Battalion, or C.B.) as humans set up camps and even homesteads on Mars. Other engineered versions of D. radiodurans could recycle wastes - producing clean water and oxygen - and perhaps even food supplements. "Its own food stock might even be Mars," Richmond suggested, giving new meaning to "living off the land." Again, the bug's genetic design might help ensure a renewable grocery store for explorers.

The ultimate step would be the popular notion of terraforming, reshaping the environment of Mars to make it more hospitable to humans. Terraforming was first performed by ancient lifeforms that converted Earth's environment from a carbon dioxide atmosphere and calcium-rich seas to the more hospitable world we have today. Because these early lifeforms spoiled their home, they now survive in what we consider to be extreme environments.

Mars, too, is considered to be an extreme environment. But with a little help from D. radiodurans, it may be made more accessible and, eventually, attractive. After all, a Seabee's motto is, "The difficult we do now. The impossible takes a little longer."

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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