Astronauts floating outside the International Space Station. (Andrei Armiagov/Shutterstock)

SAN DIEGO — When astronauts return from space missions with rashes, allergies, and weakened immune systems, the culprit might not be microgravity, but rather an environment that’s too clean. New research from the University of California San Diego shows that the International Space Station (ISS) is essentially a microbial desert compared to Earth’s environments, a finding that challenges our assumptions about what makes a healthy space habitat.

The study, published in Cell, analyzed surface samples from the ISS. Scientists can now tell whether a surface is near the space toilet, dining table, or exercise equipment simply by examining its invisible microbial and chemical residents. Mapping this data provides crucial insights for future long-duration space missions to the Moon and Mars.

The Space Station’s Hidden Ecosystem

Researchers collected 803 surface samples throughout the US Orbital Segment of the ISS during Expedition 64, creating the most extensive map ever of microbes and chemicals in a space habitat. The findings reveal that different modules of the space station have distinct microbial signatures based on how astronauts use those areas.

Unlike Earth-bound homes and buildings, which receive constant microbial inputs from the outside world, the ISS exists in complete isolation, with microbes coming almost exclusively from the astronauts themselves.

This isolation has led to a dramatic reduction in microbial diversity compared to Earth’s environments. The ISS sits at the extreme end of a spectrum that runs from pristine natural settings like rainforests to highly controlled indoor spaces.

Why Astronauts Get Sick in Space

“Future built environments, including space stations, could benefit from intentionally fostering diverse microbial communities that better mimic the natural microbial exposures experienced on Earth, rather than relying on highly sanitized spaces,” says co-first author Rodolfo Salido of the University of California, San Diego, in a statement.

The research team, led by scientists from UC San Diego and NASA’s Jet Propulsion Laboratory, found that the ISS environment is dominated by human-associated microbes, particularly those from the skin. While this might sound concerning, most of these microbes aren’t harmful and simply reflect the human presence in space.

However, this lack of environmental microbial diversity may have health implications. On Earth, exposure to diverse environmental microbes helps train our immune systems. The absence of such exposure in space might contribute to the skin rashes, allergies, and immune issues that astronauts commonly experience.

Room by Room Microbial Maps

Each module of the space station tells its own microbial story based on its function. Node 3, which houses the space toilet and exercise equipment, showed microbial signatures associated with human waste. Urine-related microbes were detected on surfaces of the toilet, while fecal microbes were found on nearby surfaces.

Meanwhile, Node 1, which serves as the dining area, contained microbes typically found in food and the human mouth. This pattern makes sense because astronauts eat there, releasing food particles and oral microbes as they talk and eat.

“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,” says co-first author Nina Zhao of UC San Diego.

Chemical Fingerprints in Space

Beyond microbes, the researchers also cataloged nearly 21,000 chemical features on station surfaces. These included everything from food components and human metabolites to industrial chemicals and cleaning products. Particularly interesting was the finding of PFAS, industrial chemicals commonly used as water and stain repellents, concentrated around the station’s treadmill, the only fabric-covered area sampled.

The research team confirmed the presence of disinfectants and cleaning products on many surfaces, despite instructions to avoid cleaning for at least four days before sampling. Interestingly, areas with higher levels of cleaning products also showed higher microbial diversity, possibly because regular cleaning creates open spots for a wider variety of microbes to colonize.

From Earth to Space: The Microbial Spectrum

The ISS shared some microbial similarities with highly industrialized indoor environments like hospitals and isolation dormitories, but had far less in common with homes in rural areas or rainforests.

Compared to most Earth samples, the ISS surfaces were lacking in free-living environmental microbes that are usually found in soil and water. The researchers say intentionally incorporating these microbes and the substrates they live in into the ISS could improve astronaut health without sacrificing hygiene. They compare their suggestion to the well-studied 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,” says study author Rob Knight.

Building Better Space Habitats

Researchers suggest that future space stations might benefit from the deliberate introduction of beneficial environmental microbes to create a more Earth-like microbial ecosystem, potentially improving astronaut health.

They also suggest design considerations like placing high bio/chemical burden areas, such as exercise or hygiene modules, at “terminal” locations to reduce cross-contamination with research areas. This approach could help maintain cleaner conditions in scientific modules while acknowledging the inevitable microbial presence in living spaces.

Monitoring Astronaut Health From a Distance

Simply by sampling surfaces, the researchers could detect signs of human viral reactivation. These viruses remain dormant in the body and can reactivate during stress. This noninvasive approach could provide early warnings about crew health issues without requiring direct medical testing. These methods could also help improve the health of people living and working in similarly sterile environments on Earth.

The ISS microbial mapping project opens a new chapter in our understanding of humans as walking ecosystems. We’ve spent decades trying to eliminate microbes from spacecraft, but this research suggests we should instead be cultivating the right microbial communities to support astronaut health.