There were also many types of bacteria that form hardy spores and are resistant to ultraviolet radiation and cleaning agents, Venkateswaran said. (Related news: "Bacteria Eat Human Sewage, Produce Rocket Fuel" [November 9, 2005].)
Knowing which organisms lurk in the supposedly sterile environments is crucial to figuring out how to eliminate them, the experts said.
"This allows us to take inventory in these environments so that NASA can come up with different ways of cleaning and sterilizing" the spacecraft, Venkateswaran said.
Left unchecked, a bacteria buildup could jeopardize space missions.
For instance, spacecraft searching for signs of life on Mars or other planets could get confusing measurements if Earthly microbes get in the way.
And if bacteria from our planet hitch a ride to other worlds, they might survive the trip and spread on those planets. (Explore a virtual solar system.)
Dead or Alive?
However, it's possible the scientists didn't find evidence of live bacteria—just genetic material left over from dead organisms.
"We do not know whether these bacteria are dead or alive at this point," Venkateswaran said. "There's no way you can tell this from the DNA fingerprinting alone."
The bacteria may have gotten into the rooms, died during sterilization, and left traces of their genetic material.
But in earlier studies, Venkateswaran and colleagues were able to get several kinds of bacteria growing—evidence that at least some of the bacteria in the clean rooms are still alive.
Mitchell Sogin of the Marine Biological Laboratory in Woods Hole, Massachusetts, said that genetic testing is a good way of spotting bacteria that don't show up in traditional lab tests.
Some of the microbes that grew included types that were once thought to be exclusive to extreme environments such as the deep sea, hot springs, and permafrost.
"For now, we want to minimize contamination" on Mars or other planets, Sogin said.
But in the long run, it may be inevitable that bacteria from Earth are spread around Mars.
"People will one day go to Mars," Sogin said, "and then it won't be possible to protect the planet."
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