Space Might Wreak Havoc on the Human Gut

This article was originally published by Undark Magazine.

From black holes to the heat death of the universe, space poses some massive dangers for humans. But as we consider long-haul space travel, there are other, smaller potential hazards that some researchers say may deserve more attention: microbes from Earth.

Astronauts face numerous known health problems in space and during flights, including bone loss, muscle atrophy, and psychological issues. And on Earth, researchers are discovering more and more how the various bacteria and other microorganisms that live inside and outside of people—the human microbiome—affect physical and mental health.

Space, of course, is an entirely different environment from Earth, with a particularly damaging form of radiation and microgravity. Although the science is far from certain, these dramatic differences may cause unexpected changes in the microbiome of astronauts. This could, in turn, result in a range of health problems, which may be more pronounced on long-haul stints in space (such as traveling to another planet).

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Still, the implications of a disrupted microbiome are poorly understood even on Earth, says David Pearce, a bioscience researcher at Northumbria University and the author of a 2022 paper exploring how a trip to Mars might affect microbes in the gut. This makes the range of related illnesses and diseases in space difficult to predict. And direct research is limited, because only about 600 people have ever been to space. Those who have taken the trip don’t typically stay long, as the average length of a trip to the International Space Station is about six months. And some researchers aren’t yet convinced that there’s enough evidence suggesting the human microbiome will change much in space at all.

Still, many researchers, including Pearce, are trying to figure out whether or not astronauts will enter dysbiosis, a state in which their microbiome changes in adverse ways. “Because they’re going to be away for a long time, will that dysbiosis become a significant problem,” he says, “or lead to them having health impacts that impair their ability to function?”

Researchers try to understand the possible effects of space on the microbiome in two places: terrestrial settings that are similar in some way to those experienced in space, and in space itself. In an example of the former, Norberto Gonzalez-Juarbe, a principal investigator with the astronaut-microbiome research group at J. Craig Venter Institute’s Infectious Diseases and Genomic Medicine Group, is looking at the microbiomes of researchers who work in the Concordia and Neumayer stations in Antarctica. He says that these locations mimic, in part, what astronauts experience in space—especially the darkness, confinement, and limited human contact.

The team plans to analyze samples from the researchers at these stations to see how the microbial composition of their gastrointestinal tracts change, and how their immune systems react to the space-station-like conditions. According to Gonzalez-Juarbe, early results show shifts in gut microbes, and the team is currently looking at the immunological data. He expects to publish the results by the end of this year.

As for studies conducted in space, there are a few. One 2019 study, for instance, compared the microbiomes of the astronaut Scott Kelly and his twin brother, Mark, after the former went to the ISS for nearly a year in 2015. The study posited that Scott Kelly’s microbiome did indeed change in space. For him, this included a temporary shift during the spaceflight: a reduction in the Bacteroidetes bacteria—the dysregulation of which has been linked to neurological, immune-system, and metabolic issues—and an increase in Firmicutes, a type of bacteria that can help break down certain starches and fibers.

In 2019, another study from the J. Craig Venter Institute looked at nine astronauts who’d spent six to 12 months in the ISS. The astronauts collected samples from various patches of their skin, nose, and tongue. The astronauts also collected stool, blood, and saliva, along with samples from various surfaces of the station and its water reservoir.

Back on Earth, the study authors extracted and sequenced the DNA from the samples to see how the astronauts’ microbiome changed over time. The study found that various skin microbes, including types of Proteobacteria, decreased in number, which the authors theorize could contribute to the common phenomenon of rashes and skin hypersensitivity among astronauts in space. The findings also suggested that the astronaut’s gastrointestinal microbiome changed, and that two types of bacteria—Akkermansia and Ruminococcus, which seem to play important roles in maintaining the digestive tract’s mucus integrity and breaking down carbohydrate—saw a five-fold decrease, though most of these changes reverted after astronauts returned to Earth.

Gut-microbiome changes can affect the metabolism of food, bone health, and even cognition, says Gonzalez-Juarbe, who was not part of the 2019 study. Longer stints in space—such as the 18 months to Mars and back—would likely compound these issues. “The saying ‘You are what you eat’ is kind of true,” he says. “Changes in the overall microbiome will have effects on your overall brain health and your cognitive health.”

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Not everyone is convinced that the human microbiome changes in space, however. Existing studies have too few subjects for any conclusions to be drawn, according to Jack Gilbert, a pediatrics professor at UC San Diego and the biology section head for the Scripps Institution of Oceanography. “With so few people up there,” he adds, “doing any studies with any statistical rigor is so hard.”

Gilbert is also skeptical of the Kelly-twin study: “We have lots of twin studies we compared over time on Earth, and they all show significant deviations from each other.”

Potentially more concerning for human health in space are microbes that could escape the body and become more dangerous, Gilbert says. A 2019 study by Gilbert and his colleagues suggests this might be the case. In March 2016, astronauts in the ISS collected samples from the station’s dining-room table. Six days later, the samples were brought back to Earth. Gilbert and his team isolated the microbes in the sample, selected two strains of the fungus Fusarium oxysporum, and sequenced their genes.

The team then compared the isolated fungi samples with 62 other strains and found that the genetics of the ISS samples differed from those of their terrestrial counterparts. The team also subjected small worms called nematodes to both samples. They found that some of the microbes from the ISS killed more of these worms.

Gilbert says that it’s possible fungus becomes more pathogenic in response to the harshness of space, although his team is working on a new study to help clarify that connection. Microbes prefer warm, moist areas, such as the environment inside the human body. So microbes that escape from that habitat onto the cold, dry surfaces—also subject to radiation and a lack of gravity—can pick up new survival skills over generations, he says. “Unfortunately,” he adds, “some of those survival strategies are associated with things like antibiotic resistance or enhanced virulence against humans.”

Gilbert notes that many of the astronauts chosen to go to space are incredibly healthy, so the chances of them getting sick from one of these rogue microbes is small. However, if someone on a long trip to Mars has a weakened immune system from food poisoning or exhaustion, he says, they could be infected by “these hard-core Mad Max survivors.”

The existing research on the human microbiome in space leaves plenty of unknowns. For instance, Nicole Buckley, team leader with the European Space Agency’s SciSpacE (or Science in Space Environment) program, notes that it’s difficult to say if any ailments in space, such as loss of sleep, are caused by microbial disruptions, or if the microbes are just contributing or reacting to other ailments.

Also unclear so far is how researchers can restabilize a person’s microbiome in space, should it be thrown out of whack to the point of illness, Pearce says. For example, fecal transplant—which involves transplanting beneficial bacteria from the stool of a healthy donor into someone who is ill—can help restore immune functions for people with certain diseases. But because microbiomes are so complex, “it’s not like administering a drug that has an outcome,” he says. “You’re administering an organism that may become established and have a desirable outcome, or it may not become established and not have the outcome you’re hoping for.”

Some of the researchers note, however, that fairly simple changes could make a difference for astronauts. Gonzalez-Juarbe says that consuming fresh fruits and vegetables and high-fiber foods can foster microbes that produce short-chain fatty acids in the stomach, which helps support the immune system. Buckley notes that pre- and probiotic foods could also help in this area.

Astronauts in space do have access to freeze-dried foods that have “normal levels of food-relevant microorganisms” but are processed to avoid containing any pathogens, according to an email from Grace Douglas from NASA’s Advanced Food Technology Project. Astronauts also receive small amounts of fresh fruits and vegetables via resupply missions. Still, Buckley says, a healthy microbiome requires limited intake of processed foods and even more fresh fruits and vegetables and high-fiber foods.

The European Space Agency is currently working on a study that adds oligosaccharides, a linked group of carbohydrates found in human breast milk, to the diet of researchers staying at the Concordia research station, in the Antarctic, for more than a year. These compounds are believed to be important in creating healthy microbiomes in babies. The study will test the oligosaccharides’ impact on the researchers’ microbiome, immune system, and mood.

There are still other fields to be explored that could further our understanding of space’s effect on the human microbiome. For instance, there’s a need for more information about individual astronauts and their microbial equilibrium—what causes their microbiome to become stable or unstable, Pearce says.

Pearce adds that astronauts may encounter familiar opportunistic pathogens—microbes that are usually benign, but that can become dangerous when a person’s immune system weakens, among other factors—like those responsible for MRSA, which is found in 2 percent of people. But there could be “unknown unknowns” in this area, he says: microbes that humans will carry into space that have the undiscovered potential to become pathogenic.

At the moment, there’s also no telling how the human microbiome would change on a long trip to Mars, compared with a relatively short stay on the ISS, Pearce says. But considering the timescale of spaceflight to the red planet—which NASA is planning for the late 2030s or early 2040s—scientists have plenty of time to better understand the microbiome’s role in astronauts’ health, he adds. Until then, Pearce says, researchers should continue using the means available to them, whether those are terrestrial studies that mimic space, studies in space itself, or simply tests that aim to better understand the microbiome of humans who are safely on the ground. “There’s no one way we’re going to get an answer for this,” he says.