Should astronauts worry about their microbiomes in space?

If you were planning a trip to Mars, someone well-versed in space travel might warn you about some potential health concerns during your journey: cancer from excessive radiation, bone loss from lack of gravity and depression from isolation. A risk unlikely to make it onto that list is the possibility that your microbiome will be disrupted. 

Scientists are just starting to study how animal microbiomes are affected by microgravity, and they’re finding significant results. 

A 2024 review paper from Weill Cornell Medicine concluded that astronauts go through a rare set of experiences in microgravity that uniquely upset their microbiomes. According to the group’s meta-analysis, space flight changes both the type and amount of microbes in their bodies, seemingly regardless of the biological differences between individuals, though they point out that the data is restricted to relatively few experiments.

A person’s microbiome is the collection of microbes, such as bacteria, that live in and on them. “We are really walking petri dishes when you think about it,” said Jamie Foster, who studies microbial communities at the University of Florida and was not involved in the Cornell paper.

Each microbiome is unique and depends on diet, environment and genetics. Microbiomes are so influential in human health that they’re “now being thought of as another organ within us,” said Joseph Bedree, a microbial strain scientist at Phare Bio whose work is cited in the paper and was not an author of the new study. 

Physical stress, like contracting food poisoning, can disrupt the normal functioning of microbe communities. Another major stress event is going into space.

The paper from Cornell surveyed the existing but minimal data about how human microbiomes behave in space, but “to study how these changes happen in the space environment is really hard,” said Foster. Since there are trillions of microbes in the human body and they’re microscopic, studying just one is hard enough, but the many limitations of conducting research in space make the endeavor that much more difficult.

So researchers often resort to scrutinizing how other animals’ microbiomes react to space.. 

Bedree, for one, has studied other animals’ microbiomes in space. In one of the studies mentioned in the Cornell paper, he looked at the microbiomes of mice that spent time aboard the International Space Station. He found that in the guts of these animals — dubbed “moustronauts” — there was an increase in certain microbes linked to bone loss.

His results were significant, but didn’t find a “smoking gun” for maintaining a healthy microbiome for human astronauts, he said.

Foster has studied a species of squid that has a sole type of microbe colonizing one of its organs. She exposed baby squid to the microbes that colonize that organ — the light organ — and then left them to grow in microgravity. Her team wanted to see if these squid and the microbes inside of them could still achieve symbiosis in space.“Luckily, yeah,” they could, she said, “so we don’t have to worry too much about the initiation of symbiosis in long-duration spaceflight.” 

The cells in human guts that interface with microbes are very similar to those of squid, “so we’re making extrapolations from this little squid to humans and trying to understand some foundational knowledge,” said Foster.

While Foster believes squid microbiomes are worth comparing to the human equivalent, Bedree is more conservative about the applicability of his research on mice. “There’s a lot of differences between rodent and human microbiomes,” he said. The specific ways in which the microbiome affects human health need to be better understood “before we get carried away with claims here.”

Eventually, Bedree hopes astronaut microbiome health will be monitored so that treatment for unbalanced microbe populations can be personalized. “There’s just not going to be a one-size-fits-all” solution, he said. It could include a mix of eating specific fiber-rich foods and taking probiotics during spaceflight.

Applying this microbiome research to practicing astronauts is important, but it can’t happen quite yet. Much more research is needed into both the processes of human microbiome functioning and how microbes are affected by space travel.

And with plans for the International Space Station to be taken down at the end of 2030, time is running out for science on live animals in microgravity. 

There’s no clear plan for what research facilities will be available to scientists in that orbit after then, “so we’ve got to come up with other ways in which we can do these kinds of experiments” in case there won’t be options to do science on animals, Foster said.

She’s considering technology called a tissue chip, which replicates groups of cells and can stand in place of an animal in future space studies. Not having to keep an animal alive would open up more access to space research for Foster — she would be able to send up small packages that require minimal oversight, rather than live organisms. She’s even pondering 3D printing cells in space eventually. But for now, small animals are her best bet for answering tough questions.

Foster is committed to continuing her research, despite the logistical unknowns, because of the good it can do for people. “Most of the science we do up in space is to help the people on Earth,” she said. She hopes her findings will eventually be used to help evolve medical therapies, maybe in the development of new medicines, that use knowledge of how microbiomes function to improve human health.