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There are currently six astronauts on board the International Space Station, but they are not alone. NASA has created a comprehensive catalogue of all the microbes on the space station, and found it is teeming with non-human life.

The agency hopes this knowledge will help it develop safety precautions for future long-term space travel, such as sending humans to Mars.

Microbes thrive on the ISS thanks to its unique combination of microgravity, radiation and tightly confined living space. A lack of competition from microbes normally found on the ground also helps certain strains to spread.

To understand which bacteria and fungi are present on the ISS, Kasthuri Venkateswaran at NASA鈥檚 Jet Propulsion Laboratory in California and his colleagues asked astronauts to take samples from eight different locations on the space station. Some were in high-traffic places, such as the bathroom and dining table, while others were in less frequented storage areas.

Using sterile wipes, astronauts took samples at three time periods 鈥� two a few months apart in 2015, and a third a year later. The specific locations were marked so that even when the crew changed, new astronauts knew exactly where to take samples from.

For the first time, the team used sequencing methods to identify microorganisms that can鈥檛 be grown in a Petri dish, which they estimate make up between 40 and 60 per cent of all microbes on the ISS. They found hundreds of thousands of strains.

鈥淭he majority of bugs we see on the ISS are benign, as we see in our home or office,鈥� says Venkateswaran. It is likely they have been there since humans first started inhabiting the space station in 2000, he says.

The researchers also found some opportunistic pathogens, including Staphylococcus aureus and Enterobacter 鈥� microbes that are often harmless, but can cause problems in people with compromised immune systems. They don鈥檛 yet know how virulent these strains are.

They are now developing ways of sequencing microbes in space instead. Sending samples back from a Mars trip wouldn鈥檛 be feasible, says Venkateswaran. 鈥淵ou鈥檒l need to do everything there.鈥�