Bugs in Space: Genes explain why salmonella grow deadlier when freed from Earth’s gravity

Bacteria that flew in a space shuttle overtook their earthbound counterparts in toxicity. The finding could have implications not only for protecting astronauts from sickness as spaceflights become longer and more frequent but also for understanding bacteria on Earth.

Previous experiments using a spaceflight stimulator had hinted that bacteria might grow more virulent in the absence of gravity. To see whether real space flight would produce a similar result, a team led by Cheryl Nickerson of Arizona State University in Tempe sent samples of Salmonella typhimurium, a leading cause of food poisoning, into orbit on the Space Shuttle Atlantis in September 2006. The bacteria grew in closed tubes, and shuttle crew members tended to them—controlling their temperature, for example, or adding nutrients. Meanwhile, scientists on the ground mirrored the astronauts’ procedures on cultures kept in a room designed to reproduce conditions on the shuttle. The only difference: gravity.

When the space shuttle landed, the scientists compared the two sets of pathogens by testing their effects on mice. They found that animals injected with S. typhimurium that had traveled to space died faster than those injected with bacteria that hadn’t made the trip.

To explore this difference in virulence, the team analyzed what genes were active in each group of bacteria.

The researchers found that 167 genes showed more than twofold differences in activity, either higher or lower, between bacteria grown in the shuttle and those grown on Earth. Moreover, 64 of the genes were related, all of which were involved in controlling a protein called Hfq, which is known to help bacteria cope with changing external conditions. The results, which appear online and in an upcoming Proceedings of the National Academy of Sciences, suggest that Hfq has a surprising role in mediating bacterial virulence in space, and perhaps on Earth.

Some aspects of life on a spaceship aren’t that alien to organisms on Earth. A liquid medium in low gravity provides conditions resembling those inside the human body. Bacteria grown in labs are often shaken vigorously in flasks to speed their growth. Such treatment may approximate conditions in fast-moving blood flows, but not the more-sedate settings within urinary or gastrointestinal systems, where bacteria also would have adapted to survive.

The similarity of weightlessness to some internal environments suggests an evolutionary reason why many kinds of bacteria might be adapted to ramp up virulence in space, Nickerson says.

Microbiologist David W. Niesel of the University of Texas Medical Branch in Galveston calls the study “the ultimate validation” of older results from flight stimulators. He adds that he isn’t too surprised by the fact that bacteria behave differently in space and in a lab on Earth, “because the way bacteria have persisted for eons is to be completely adaptable.”

From the Nature Index

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