The ISS’s Sterile Environment Could Be Making Astronauts Sick

Living in space presents many challenges, from extreme radiation to bone loss. However, an often-overlooked factor is the microbial environment inside spacecraft.

On Earth, humans exist within a vast and complex microbial ecosystem, but space habitats like the International Space Station (ISS) are highly controlled, enclosed environments where microbial diversity is drastically reduced.

This isolation poses both advantages and risks. While fewer pathogens may seem beneficial, the absence of beneficial microbes can weaken astronauts' immune systems and contribute to health problems.

A new study led by researchers from the University of California, San Diego, the California Institute of Technology, and NASA, published in the journal Cell, provides the most comprehensive analysis of the ISS's microbial and chemical landscape to date.

The study, titled "The International Space Station Has a Unique and Extreme Microbial and Chemical Environment Driven by Use Patterns," reveals how human activity shapes the space station’s microbiome and highlights potential health risks for astronauts on long-term missions.

To understand how the ISS environment compares to those on Earth, researchers analyzed 803 surface samples across the station’s United States Orbital Segment (USOS). This dataset was nearly 100 times larger than previous studies.

The samples were used to create a high-resolution 3D model of microbial and chemical distribution across the station’s surfaces. This allowed researchers to visualize how different areas of the ISS accumulate specific microbes and chemicals over time.

The results showed that the station’s microbiome is overwhelmingly dominated by human-associated bacteria, primarily from skin. The study found stark differences between microbial communities in different ISS modules, depending on their primary function.

For example, food preparation and dining areas contained microbes linked to food, while hygiene spaces harbored higher levels of bacteria associated with human waste. This suggests that astronaut activity significantly influences microbial distribution, just as it does in homes and hospitals on Earth.

One surprising finding was that ISS microbial diversity is lower than nearly any built environment on Earth, including hospitals and industrial buildings. This is likely due to rigorous cleaning protocols and the station’s isolation from natural microbial sources.

However, this extreme sterility might not be ideal. On Earth, exposure to a diverse range of microbes helps maintain immune system balance. Without it, astronauts may be at greater risk for inflammatory conditions, allergies, and immune dysfunction.

While reducing harmful bacteria is crucial in space, the complete absence of beneficial microbes could be counterproductive. Studies have linked reduced microbial exposure to chronic inflammatory diseases in highly sanitized environments on Earth. In space, astronauts frequently experience rashes, allergies, and immune dysfunction, including the reactivation of latent viruses and an increased risk of infections.

"Truly sterile environments—completely devoid of microbial life—are exceptionally rare on Earth," said Rodolfo Salido Benítez, the study's co-first author and Director of Laboratory Automation at the Knight Lab, UC San Diego. "In space, excessive chemical decontamination may be counterproductive to maintaining a healthy ecosystem."

Beyond individual health, microbial life in space could also pose risks to spacecraft integrity. Some bacterial species found on the ISS have demonstrated increased resistance to antibiotics, heightened virulence, and biofilm formation.

These adaptations could lead to corrosion of spacecraft materials, equipment malfunctions, or heightened astronaut health risks. Although current evidence suggests that space does not necessarily promote the evolution of more dangerous microbes, ongoing monitoring is critical.

The study underscores the need for a new approach to microbial management in space. Instead of aiming for extreme sterility, researchers suggest fostering a controlled but diverse microbial environment that mimics Earth’s natural exposures. Introducing beneficial microbes could help maintain astronaut health while mitigating the risks associated with antibiotic resistance and biofilm formation.

"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," Salido Benítez noted.

The findings have implications beyond space travel. Insights gained from the ISS could help improve microbial risk management in hospitals and other highly sanitized environments on Earth. By understanding which microbes are necessary for health and how to maintain a balanced microbial environment, researchers hope to design healthier living spaces both in space and on the ground.

NASA and other space agencies are already exploring new methods for microbial monitoring. While traditional culture-based methods have been used to assess microbial safety thresholds, these approaches fail to capture the full diversity of microbes present.

Advances in metagenomics and in-flight DNA sequencing now allow astronauts to analyze microbial samples in real-time, paving the way for more adaptive and precise microbial management strategies.

As human space exploration extends beyond the ISS to the Moon and Mars, maintaining a stable and healthy microbial ecosystem will be essential. Future habitats must balance sanitation with the need for beneficial microbial diversity, ensuring that astronauts can thrive in space without unnecessary health risks.

Note: Materials provided above by The Brighter Side of News. Content may be edited for style and length.

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