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When the Starship Enterprise went roaming through the galaxy, the captain and crew had to be vigilant. There might be Klingons, Romulans, disembodied intelligences composed of pure energy. What they never seemed to worry about was the threat of festering scum underneath the toilets or behind the sinks. But maybe they should have, according to Cynthia Collins and Jon Dordick.

Collins and Dordick are scientists at Rensselaer Polytechnic Institute in Troy, NY. — a microbiologist and a nanotechnologist, respectively — and when the space shuttle Atlantis lifts off on Friday for the shuttle program's final flight, their experiment could be crucial to the health of future astronauts. There's evidence from earlier spaceflights that virulent bacteria such as E. coli and salmonella can grow stronger and faster in microgravity. Given enough time, they can form biofilms, slimy mats of bacteria that are highly resistant to antimicrobials. If the films get inside the body, riding on catheters or other medical equipment, the human imune system may be powerless against them. That's true even when the immune system is functioning at its best, but in space, for reasons nobody fully understands, the body's disease defenses are weaker.

"It's anecdotal," admits Dordick, "but over the years, Russia's Mir space station evidently became coated with biofilms."

On a week-long shuttle mission, the danger is minimal. But biofilms could pose a huge problem for passengers aboard the International Space Station or, someday, on a voyage to Mars. "More than 60 percent of infections people get in hospitals," says Collins, "are caused by biofilms." If it's impossible to keep bacteria out of a hospital environment that's supposed to be spotless, it's equally hard to keep them out of space. "You can't very well sterilize an astronaut," says Dordick.

So Dordick and Collins are trying to figure out a subtler approach. When Atlantis flies, it will be carrying 16 small reactor vessels (sealed, of course), each containing eight vials full of Pseudomonas aeruginosa or Staphylococcus aureus bacteria, the two most common causes of hospital-acquired infections (the latter has evolved an especially nasty antibiotic-resistant strain known as MRSA). They've also created an identical set of vessels that will stay behind on Earth

By comparing how the space- and ground-based samples grow, Collins, Dordick and several colleagues hope to get a handle on what it is about microgravity that makes bacteria so happy. "We're modulating oxygen, nutrients and other factors," says Collins, in order to come at the question in as many different ways as possible.

They're also testing out a bacteria-fighting technology developed in Rensellaer's Center for Biotechnology and Interdisciplinary Studies, which Dordick directs. "We've created glass slides coated with paint containing an enzyme called lysostaphin," he says, which has been shown to inhibit the growth of staph biofilms in ground-based tests. The Atlantis experiment will tell them how well it works in space.

Ideally, the Rensselaer team would re-run the experiment many more times, using different bacteria and different bacteria-inhibiting coatings to find out not just what works, but what works best. They're not limiting their interest to studying only harmful bugs, either. "The body has thousands of bacteria we absolutely need to have to stay healthy," says Collins. "We need to maintain that balance if we're going to do long-term spaceflight."

But since this is the last shuttle mission, the rest of the experiments on the scientists' long wish list will remain undone. "You can sort of mimic microgravity on the ground," says Dordick, "but it's really not the same." In principle, they could continue the experiments on the space station, but the Russian Soyuz spacecraft that will now be the only way to get there doesn't have a lot capacity to haul experiments up and down. "It's something we're looking at a little bit," says Collins, "but we're not counting on it."

She's got higher hopes for the burgeoning commercial space industry, which may give scientists another route into space. That won't happen for a while, but if humans are ever going to explore the solar system in person, these experiments will need to be done. Nasty infections are bad enough when they happen in hospitals. They'd be incomparably worse showing up halfway through a six-month passage to Mars.

• Michael D. Lemonick