ROBERT SIEGEL, host:

Now, a story about germs in space. Astronauts are normally kept in quarantine before launch, partly to keep them free of germs so that they don't get sick in orbit. But despite NASA's best efforts, sometimes bugs do hitch a ride off the planet.

And as NPR's Nell Greenfieldboyce reports, a new study suggests that spaceflight can make those germs more dangerous.

NELL GREENFIELDBOYCE: Last year, bacteria didn't have to sneak on to space shuttle Atlantis. Scientists put them onboard and watched them blast off.

Unidentified Man: Atlantis, Houston, go and throttle up.

GREENFIELDBOYCE: Of course, the bacteria were safety sealed up in tubes and not just floating around the cabin.

Dr. CHERYL NICKERSON (Biologist, Arizona State University): It was never any danger to the crew at any time during this experiment.

GREENFIELDBOYCE: Cheryl Nickerson is a biologist at Arizona State University. She wanted to see how spaceflight affects salmonella, a germ that causes diarrhea and food poisoning.

Dr. NICKERSON: Most of us have experienced a salmonella food-borne infection, and it's not pleasant.

GREENFIELDBOYCE: While the shuttle orbited Earth, one of the astronauts turned a crank on a special canister. This mixed the bacteria with some food so they started to grow. Then the shuttle headed home.

Dr. NICKERSON: These samples were recovered incredibly quickly off of the shuttle upon landing.

GREENFIELDBOYCE: Nickerson says, within just a couple of hours, before the bacteria could readjust to life on Earth, the scientists used the bacteria to infect mice. These mice ended up doing much worse than animals infected with bacteria grown on the ground.

Dr. NICKERSON: They got sick quicker and they ultimately succumbed to the infection quicker.

GREENFIELDBOYCE: Nickerson says this is the first time anyone has shown that bacteria can become more dangerous after spaceflight. Previous studies had shown that some bacteria grow faster in space or seem better able to resist antibiotics.

Dr. NICKERSON: So we had a lot of basic fundamental knowledge that spaceflight can change a lot of properties of microbial cells, but we didn't have any mechanistic understanding at the molecular level of why that was happening.

GREENFIELDBOYCE: To answer the why question, they looked for genetic changes in the salmonella grown in space. Overall, they found changes in the activity of over 150 genes. Nickerson says many people find it surprising that microbes even care about weightlessness.

Dr. NICKERSON: Why would you expect that microgravity would be an environment the cells would respond to?

GREENFIELDBOYCE: She believes they're actually reacting to the way that low gravity changes how fluid pushes against their cell membranes.

Dr. NICKERSON: And I'm kind of giving them human properties here, but they know when that force is low and they know when that force is high.

GREENFIELDBOYCE: The study is being published today in the proceedings of the National Academy of Sciences. It intrigued researchers who study how spaceflight affects the health of astronauts.

Millie Hughes-Fulford works at the VA Medical Center in San Francisco. Over a decade ago, she went up on the shuttle to do experiments. Like all astronauts, she was put into strict quarantine before flying.

Dr. MILLIE HUGHES-FULFORD (Deputy Associate Chief of Staff, The VA Medical Center): You can't go into a drugstore and buy a pack of gum. In training, only certain people can come near you, and those people have been certified to be healthy.

GREENFIELDBOYCE: But, she says, astronauts do sometimes get sick in space. And her work suggests that low gravity can weaken people's immune system. She says certain immune cells just don't act normally in space.

Dr. HUGHES-FULFORD: They can't just turn on like they normally do when they're presented with a challenge.

GREENFIELDBOYCE: So she says the new bacteria study suggests that astronauts could face a double whammy - a weaker immune system trying to fight off stronger bacteria.

Nell Greenfieldboyce, NPR News.

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