Scientists have shown how bacteria in space can gain virulence.
When Salmonella typhimurium food bugs were flown in special flasks on the shuttle, they were found to alter the way they expressed 167 genes.
The bacteria were almost three times as likely to kill infected mice compared with standard samples held on Earth.
The study, presented in Proceedings of the National Academy of Sciences, is said to have important implications for astronauts going to the Moon or Mars.
S. typhimurium is one of the more difficult food bugs to treat with antibiotics, and long spaceflight missions would need to take care that good hygiene standards were maintained.
"Wherever humans go, microbes go; you can't sterilise 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," Cheryl Nickerson, from the Center for Infectious Diseases and Vaccinology at Arizona State University, US, told the Associated Press.
Her research team says the space bacteria changed in response to the microgravity conditions they experienced on their 12-day Atlantis orbiter flight in September 2006.
"These bugs can sense where they are by changes in their environment. The minute they sense a different environment, they change their genetic machinery so they can survive," Professor Nickerson said.
The research team discovered that a large number of the genetic changes appeared to be regulated by a protein known as Hfq.
The group says that a drug developed to target Hfq could help protect astronauts from infectious disease during spaceflight and people on Earth.
Currently, no vaccine exists for Salmonella food-borne infections in humans. The new study may offer possible solutions, the team believes.
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