More microbes on super-sterile ISS could improve astronaut health

Astronauts often experience immune dysfunction, skin rashes and other inflammatory conditions while travelling in space, with new research led by the University of California, San Diego suggesting that these issues could be due to the excessively sterile nature of spacecraft.

As published in the journal Cell, the study showed that the International Space Station (ISS) has a much lower diversity of microbes compared to human-built environments on Earth, and the microbes that are present are mostly species carried by humans onto the ISS. Its publication came shortly before the return to Earth of the two NASA astronauts who spent nine months stranded on the ISS.

The researchers collaborated with astronauts who swabbed 803 different surfaces on the ISS — around 100 times more samples than were taken in previous surveys. Back on Earth, the researchers identified which bacterial species and chemicals were present in each sample. Then, they created maps illustrating where each was found on the ISS and how the bacteria and chemicals might be interacting.

The team found that overall, human skin was the main source of microbes throughout the ISS. Chemicals from cleaning products and disinfectants were present ubiquitously throughout the station. They also found that different rooms within the ISS hosted different microbial communities and chemical signatures, and these differences were determined by the module’s use — so dining and food preparation areas contained more food-associated microbes, whereas the space toilet contained more urine- and faecal-associated microbes and metabolites.

“We noticed that the abundance of disinfectant on the surface of the International Space Station is highly correlated with the microbiome diversity at different locations on the space station,” said co-first author Nina Zhao.

When they compared the ISS to different human-built environments on Earth, the researchers found that the ISS microbial communities were less diverse than most of the samples from Earth and were more similar to samples from industrialised, isolated environments, such as hospitals and closed habitats, and homes in urbanised areas.

Compared to most of the Earth samples, the ISS surfaces were lacking in free-living environmental microbes that are usually found in soil and water. The researchers suggested that intentionally incorporating these microbes and the substrates they live in into the ISS could improve astronaut health, noting the beneficial impacts of gardening on the immune system.

“There’s a big difference between exposure to healthy soil from gardening versus stewing in our own filth, which is kind of what happens if we’re in a strictly enclosed environment with no ongoing input of those healthy sources of microbes from the outside,” said co-author Rob Knight.

In the future, the researchers hope to be able to detect potentially pathogenic microbes and signals of human health from environmental metabolites. They say that these methods could also help improve the health of people living and working in similarly sterile environments on Earth.

“If we really want life to thrive outside Earth, we can’t just take a small branch of the tree of life and launch it into space and hope that it will work out,” said co-first author Rodolfo Salido. “We need to start thinking about what other beneficial companions we should be sending with these astronauts to help them develop ecosystems that will be sustainable and beneficial for all.”

Image caption: Demonstration of 3D data visualisation for surface sampling. Figure created in part with BioRender by Lab, K. (2025) (shared under CC BY 4.0)