It seems counterintuitive that researchers can learn about how bacteria behave in our bodies on Earth by putting them in an environment as artificial as a test tube on a spaceship, says Jeanne Becker, associate director of the National Space Biomedical Research Institute, in Houston. "Looking at it from the perspective of the bacteria," she says, "they want to be able to survive in a stressful environment"--whether it's microgravity, an assault by the immune system, or the presence of an antibiotic. The way bacteria respond to a stressful environment--by making more or less of a particular protein, for example--can point researchers toward biochemical pathways that novel therapeutics might target.

Spaceflight also alters the genetic activity of human cells. "We evolved in a one-gravity environment," says Becker. "There are fundamental changes when you take gravity away." One study showed that spaceflight caused changes in the expression of more than 1,600 genes in human kidney cells grown in culture. Becker says that many more studies of the molecular effects of space conditions on cells are forthcoming, as researchers take advantage of the International Space Station.

Nickerson's group is soon to publish papers on the effects of spaceflight on a strain of yeast and on Pseudomonas aeruginosa, the bacterium that caused the only severe infection of an astronaut to date, in 1970. To make a successful reentry into the atmosphere after an onboard explosion, the Apollo 13 astronauts retreated from the main part of their craft into their lunar module, where they faced a short supply of oxygen, power, and water. Under these extreme conditions, astronaut Fred Haise developed an aggressive prostate infection caused by Pseudomonas and was seriously ill for a few weeks after returning home.

Most infections of astronauts in space have been mild--Haise's case is the exception--and none was caused by Salmonella. Nickerson says that she chose to study Salmonella because earlier experiments that simulated microgravity and other space conditions suggested that the bacterium's virulence might increase in space. "While Salmonella has never been isolated from spacecraft, it's an important reason food destined for the International Space Station gets disqualified," she says.

"The fact that at least one nasty bug becomes demonstrably more virulent in spaceflight introduces a hazard that might before have been underappreciated," says Kim Prisk, a professor of medicine at the NASA Lab at the University of California, San Diego. "It does make one wonder about how aggressive the medical treatment of an infected subject in space would need to be."

More important, say Prisk and Becker, is the Arizona researchers' identification of the global regulator of virulence that might lead to treatments for Salmonella infections. "It's a good example of how spaceflight-oriented research can lead to potentially important and beneficial results here on the ground," says Prisk.