WASHINGTON (AP) -- It sounds like the plot for a scary B-movie: Germs go into orbit on a spaceship and come back stronger and deadlier than ever.

But it really happened.

The germ: Salmonella, best known as a culprit in food poisoning.

The trip: Space shuttle mission STS-115, September 2006.

The reason: Scientists wanted to see how space travel affects germs, so they took some along -- carefully wrapped -- for the ride.

The result: Mice that were fed the space germs were three times more likely to get sick, and died more quickly, than mice fed identical germs that had remained behind on Earth.

"Wherever humans go, microbes go -- you can't sterilize humans. Wherever we go, under the oceans or orbiting the earth, the microbes go with us, and it's important that we understand ... how they're going to change," explained Cheryl Nickerson, an associate professor at the Center for Infectious Diseases and Vaccinology at Arizona State University.

Nickerson added, in a telephone interview, that learning more about changes in germs has the potential to lead to novel new countermeasures for infectious disease.

She reports the results of the salmonella study in Tuesday's edition of Proceedings of the National Academy of Sciences.

The researchers placed identical strains of salmonella in containers and sent one into space aboard the shuttle, while the second was kept on Earth, under similar temperature conditions to the one in space.

After the shuttle returned, mice were given varying oral doses of the salmonella and then were watched.

After 25 days, 40 percent of the mice given the earthbound salmonella were still alive, compared with just 10 percent of those dosed with the germs from space. And the researchers found the amount of bacteria it took to kill half the mice was three times larger for the normal salmonella than for the space germs.

The researchers found 167 genes had changed in the salmonella that went to space.

Why?

"That's the 64 million dollar question," Nickerson said. "We do not know with 100 percent certainty what the mechanism is of space flight that's inducing these changes."