Humans weren't built for space travel – the harsh environment messes with our vision, circulation, muscles and bones, and increases our risk of cancer and diabetes. The immune system also takes a beating, leaving us more vulnerable to infection. Astronauts have now tested a new antimicrobial coating on board the International Space Station (ISS), and found it kept the space-faring bugs at bay for well over a year.

Bacteria are resilient little critters, able to adapt to the most extreme environments – and few environments are as extreme as space. It's been found that microgravity helps microbes evolve faster, which doesn't bode well for the poor immune-compromised astronauts that are stuck in there with them.

Since extended spaceflight seems likely to be more and more common in humanity's future, new antimicrobials need to be developed and tested to reduce the risk of infection. In this study, astronauts onboard the ISS tested out a new antimicrobial coating called AGXX.

"AGXX contains both silver and ruthenium, conditioned by a vitamin derivative, and it kills all kinds of bacteria as well as certain fungi, yeasts and viruses," says Elisabeth Grohmann, senior author of the study. "The effects are similar to bleach – except the coating is self-regenerating so it never gets used up."

The team tested the coating on the space station's toilet door, which understandably sees a lot of bacterial action. The test area was examined periodically afterwards. After six months, no bacteria at all were found on the AGXX surface. At the 12-month mark, only nine bacteria colonies were found, and after 19 months, only three. That's 80 percent fewer bacteria than would grow on bare steel.

For comparison's sake, the researchers also tested a regular silver coating, which is a common antimicrobial material. That also inhibited bacterial growth but not quite as well, reducing levels by 30 percent over bare steel.

Although the AGXX coating only ever needs a single application, its effectiveness does seem to wane over time. But, the team says, that's not so much because it wears out but because dust and dead cells build up on the surface, forming a safe layer that new microbes can survive on. An annual spot of cleaning might help extend its lifespan.

The team also conducted an analysis of which species were present on the ISS, and the news was surprisingly positive. No serious human pathogens were detected on any surface. That said, most species were found to be resistant to at least three antibiotics, and were able to share those genes responsible for antibiotic resistance around. Additionally, all of them could form biofilms, which helps their survival odds.

Understanding the microbiome of spacecraft is important in a future that's increasingly focused on the stars. The team says it plans to continue testing the effectiveness of AGXX.

"Immunosuppression, bacterial virulence and therefore infection risk increase with duration of spaceflight," says Grohmann. "We must continue to develop new approaches to combat bacterial infections if we are to attempt longer missions to Mars and beyond."

The research was published in the journal Microbiology.