It almost goes without saying that space is a pretty harsh environment. A NASA twin study showed life in space changes gene expression in humans, and tests have shown that it even stresses seedlings. Now researchers have examined what effects that environment might have on bacteria, and found that the bugs are mutating in response – but that probably isn't cause for concern.

Bacteria are hardy little critters, regularly turning up in places we once thought were too extreme for life. They adapt very quickly when exposed to new dangers – one of the most worrying examples being their fast-evolving resistance to antibiotics, which may be considered the number one threat to human health in the near future. These important drugs wipe out all but the most resilient bacteria, so that eventually the only remaining strains are the so-called "superbugs."

There has been some concern that superbugs could also be forged in the fires of space. Although the ISS is a tightly-controlled environment, it's virtually impossible to keep it completely clean – bacteria inevitably hitch a ride into space on astronauts and equipment. Samples of spacefaring bugs have been collected and studied, and kept in a database managed by the National Center for Biotechnology Information.

The new study, by researchers at Northwestern University, examined the genomes of bacteria found on the ISS and compared them to Earthly specimens. While there were differences between bugs living in those wildly different environments, they weren't becoming any more hazardous to human health.

"There has been a lot of speculation about radiation, microgravity and the lack of ventilation and how that might affect living organisms, including bacteria," says Erica Hartmann, lead researcher on the study. "These are stressful, harsh conditions. Does the environment select for superbugs because they have an advantage? The answer appears to be 'no'."

The two strains examined were Staphylococcus aureus and Bacillus cereus. The former lives on human skin and while most of the time it's fairly harmless, one form, Methicillin-resistant S. aureus (MRSA), is high on the World Health Organization's superbug priority list. The second strain, commonly found in soil, is less of a worry for human health.

But while the team found numerous genetic changes in both strains in space, they weren't becoming more dangerous. According to the researchers the bugs are more concerned with their own wellbeing than ours, and are just trying to make the best of a bad situation.

"Based on genomic analysis, it looks like bacteria are adapting to live – not evolving to cause disease," says Ryan Blaustein, first author of the study. "We didn't see anything special about antibiotic resistance or virulence in the space station's bacteria."

The research was published in the journal mSystems.