For nearly 25 years, the International Space Station (ISS) has been continually inhabited, making it one of the most remote human outposts ever established. Orbiting in the sparse expanse of low-Earth orbit, the ISS has hosted approximately 270 human occupants along with various animal visitors and the accompanying microbes that have traveled with them into space.

In this unique environment, these microbes have begun to evolve. Subject to cosmic radiation, bacteria have developed new DNA repair mechanisms. Some have acquired resistance to antibiotics and disinfectants, or have undergone other transformations that increase their pathogenic potential.

“To take care of us humans, we have to take care of our human microbes. And that’s going to be a very interesting challenge.” —Martin Blaser, microbiologist

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“This environment is incredibly extreme,” notes Rodolfo Salido, a bioengineer at the University of California, San Diego. The microbial inhabitants of the ISS can have a direct impact on astronaut health. To chart the microbial landscape of the space station, Salido and his team dispatched swabs to space, where astronauts collected samples from hundreds of surfaces. Their study, presented in the journal Cell on Thursday, reveals a three-dimensional map of microbial diversity on the ISS, showing a lack of many bacterial types typically encountered on Earth, which could be crucial for astronaut health. In future prolonged space missions, astronauts might need closer microbial companionship for better health.

“To take care of us humans, we have to take care of our human microbes. And that’s going to be a very interesting challenge” in space exploration, states Martin Blaser, a microbiologist at Rutgers University, who did not participate in the recent study.

In December 2020, Salido and his team, in collaboration with NASA’s Jet Propulsion Laboratory, sent around 1,000 redesigned, sterilized sampling devices to the ISS. These swabs were tailored for space use—a lesson Salido learned during a visit to a replica of the ISS in Houston, where astronaut Michael Barratt highlighted the unsuitability of their regular, overly large and flammable swabs for space usage.

Once the redesigned swabs reached the ISS, astronaut and microbiologist Kathleen Rubins, along with other crew members, spent a total of 24 hours collecting samples from various surfaces within the U.S. segment of the station. They gathered 803 swabs in total, which were returned to Earth in October 2021 for analysis. The team identified the genes and chemical by-products present, indicating that most bacteria on the ISS originated from human skin, particularly species like Staphylococcus, with notably few bacteria from Earth’s soil and water.

“It’s similar to any other indoor environment you might find yourself in,” explains Blaser. “We continuously shed microbes from our skin,” and indoor areas, especially those with poor air circulation, tend to have more microbial presence than outdoor environments. Certain indoor locations exhibit a more pronounced microbial imbalance than others. In previous research, Blaser and his team analyzed microbial samples from rural and urban homes across South America—from isolated Amazonian villages to the bustling city of Manaus, Brazil—revealing that more isolated living spaces tend to have reduced microbial diversity.

However, the ISS is an extraordinary case. When comparing their ISS samples with those from South American homes and other terrestrial sites like hospitals, the researchers found the space station had significantly lower microbial diversity. One notable comparison, according to environmental chemist Haoqi Nina Zhao from U.C. San Diego and co-lead author of the study, was with a COVID-19 isolation dormitory, which closely resembled the ISS’s microbial environment.

The potential health implications of this microbial imbalance for astronauts include skin rashes and immune system issues, though these are still under investigation. “As we transition into more artificial settings, on Earth or in space, we’re severing our traditional microbial relationships,” Salido comments. “Our immune systems haven’t adapted to this new reality yet.”

While some studies suggest that low microbial diversity might increase the risk of immune dysfunction, particularly in children whose microbiomes are still developing, it’s not yet clear how these findings apply to healthy adults temporarily living in space, according to Blaser. This could change if humans begin longer-duration stays or establish colonies beyond Earth.

“I’d be curious how the microbiomes of babies born in such conditions would evolve,” Blaser muses. “That will be a crucial question for humanity if we start colonizing other planets.”

In our future among the stars, we might need to deliberately bring along beneficial microbes while managing less favorable ones. Instead of traditional chemical disinfectants, which can promote resistance, the study suggests investigating probiotic-based cleaning solutions that introduce benign bacteria to outcompete dangerous ones.

Although some indoor environments might lack essential microbes, the solution isn’t to abandon hygiene. “It’s not about forgoing cleanliness,” Salido clarifies. Moving forward, it’s about developing methods that allow our built environments to incorporate the beneficial microbes with which we have co-evolved.