For astronauts toiling in the close quarters of the International Space Station or on a shuttle to Mars, an ordinary germ would be risky enough. But a recent experiment published in the Proceedings of the National Academy of Sciences has shown that a microbe can turn even more dangerous in space than on Earth. In that study, a bacte­rium particularly nasty for humans—salmonella—was shown to become more virulent after just 83 hours of growing in space.

The experiment on the space shuttle Atlantis was designed to explore how a lack of gravity affects disease-causing microbes in space. Astronauts aboard the space shuttle grew the salmonella, and back on Earth researchers used it to infect a group of mice. For comparison, bacteria grown in a laboratory on Earth in normal gravity infected another group of mice. The mice infected with the space-grown germs had a mortality rate almost three times higher than that of mice given germs grown in normal gravity.

Researchers noticed that while on board the space shuttle, the salmonella encased themselves in a biofilm, a protective coating that is notoriously resistant to anti­biotics. Several follow-up experiments on space shuttle flights over the next few years will look to see whether other bacteria undergo similar changes in virulence in microgravity.