What a NASA Study on Astronaut Microbiomes Reveals About Our Health
Ultra-sterile living conditions may contribute to common astronaut illnesses, including gut, skin, and immune-system problems
Spaceflight is hard on the human body. The absence of gravity can induce a form of nauseating motion sickness known as space adaptation syndrome. As time passes, weightlessness can also cause muscle wasting, bone deterioration, and other health problems.
NASA and its sister space agencies around the world have long recognized these health threats, and they’ve developed effective countermeasures. But as they’ve learned to manage the challenges of zero-gravity environments, other concerns have emerged.
According to a 2016 NASA-led study in the International Journal of General Medicine, time spent in space rapidly perturbs the human immune system. Nearly every molecule and marker of healthy immune operation — from the signaling activity of T cells to the production and action of cytokines — becomes dysregulated in space, and this dysregulation seems to contribute to a number of well-documented health problems, that study found.
For example, astronauts in space often develop skin rashes, allergies, and infections of the upper-respiratory and urinary tracts. Meanwhile, long-dormant viral infections — such as those caused by the assorted herpes viruses — are prone to flare. These flares can cause cold sores, shingles, mononucleosis, and other health problems, including some that the astronauts can pass among one another.
Not all of these immune issues go away when the astronauts return to Earth. There’s concern that extended spaceflight — due in part to immune-related changes or impairments — could raise an astronaut’s long-term risks for various cancers, heart disease, or gut disorders such as inflammatory bowel disease (IBD).
There’s are many aspects of space travel that could cause an astronaut’s immune system to go haywire. But NASA researchers are increasingly focusing their attention on the human microbiome — the trillions of bacteria that live on and inside our bodies, and that play an essential part in the operation of our immune systems.
What they’ve found already may hold important lessons for all of us back here on Earth.
The human body is home to multiple distinct ecosystems of microbes, each of which is composed of many different species of bacteria. There is a skin microbiome, a gut microbiome, a nose microbiome, and several others. “In each of those ecosystems, microbes play different roles in human health,” says Hernan Lorenzi, PhD, an assistant professor in the Department of Infectious Diseases at the J. Craig Venter Institute in Maryland.
Lorenzi has led some of NASA’s research projects into the microbiome effects of long-term space travel.
For a 2019 study in Scientific Reports, he and his colleagues collected skin, nose, tongue, and gut bacteria samples from nine astronauts, each of whom spent between six months and one year aboard the International Space Station (ISS). “Basically, we found that during the mission many of these microbiomes changed, and some of these changes correlated with changes in immune responses among the astronauts,” he says.
“We found changes in the intestinal microbiome that were associated with the kind of immune dysregulation that, if it becomes chronic, can cause many problems, including cancer.”
For example, Lorenzi’s team found a drop in Proteobacteria on the astronauts’ skin. “Proteobacteria are known to play a protective role against hypersensitivity reactions like allergies,” he explains. Extended spaceflight often induces skin rashes, skin infections, and other dermatological conditions. “These are some of the most common adverse experiences astronauts have, and it may be that those changes in bacteria on the skin are contributing to that,” he says.
Lorenzi and his colleagues also observed troubling shifts within the astronauts’ gut microbiomes.
“We found changes in the intestinal microbiome that were associated with the kind of immune dysregulation that, if it becomes chronic, can cause many problems, including cancer,” he says. These changes included a reduction in the abundance of three species of bacterium — Fusicatenibacter, Pseudobutyvibrio, and Akkermansia — that are associated with lower levels of gut inflammation. “We also saw a reduction of some bacteria that are known to help the intestinal epithelium” — that is, the outer wall of the GI tract — “to remain healthy,” he says. (More and more research suggests that epithelial dysfunction is closely linked to IBD, a cluster of gut diseases that have surged in the U.S. and elsewhere during the past 20 years.)
The many ways that bacteria influence human health are too numerous to list here. But science has established beyond all doubt that, far from being passive passengers, our bacteria help to shape and regulate the action of our immune systems, our nervous systems, and much else.
The composition of a person’s gut microbiome appears to be especially important, and imbalances or irregularities in its bacteria populations are linked with many illnesses — from gut disorders and autoimmune diseases to mental-health problems. (Research has also linked gut microbiome factors to a person’s Covid-19 risks.)
In most ways — but not all — the astronauts’ microbiomes returned to their baseline compositions within two months of their return to Earth. But what was it about space travel that threw them off?
“That’s a good question,” Lorenzi says. “We think that diet could play a role, although the astronauts are eating a diet that’s designed to keep them healthy.” He says that stress, the lack of gravity, and circadian-cycle disruptions could all partly explain the problematic microbiome shifts.
But he also says that the sterile environment on the International Space Station was likely a contributing factor. “People think that, well, it’s healthier to stay as clean as possible and to try to sterilize everything, but I think our work shows that being in an ultra-clean, ultra-sterile environment is not a good thing,” he says.
The idea that our bodies or spaces could be too clean is not a new one.
The “hygiene hypothesis” — the theory that our health may depend on our interactions with bacteria, and maybe even with some germs — has been kicking around and gaining steam since at least 1989. That year, the epidemiologist David Strachan observed a link between “unhygienic contact” and a reduced risk for hay fever, eczema, and other immune-mediated illnesses in kids.
For a while, some experts rejected the hygiene hypothesis because no one could explain how it worked. We thought that germs were bad (of course, some are), and we viewed the body’s bacteria as opportunistic stowaways, as opposed to essential partners. But science’s recognition of the microbiome’s role in human health, a recognition that has coalesced only in the last 10 or so years, provided that mechanistic missing link between our health and our interactions with bacteria.
“If you’re in an environment that is ultra clean, I think you can expect that your microbiome will become less rich because you don’t have this constant reinfection through contact.”
At this point, researchers have solidly established that early life exposures to “good” bacteria help to populate the body’s various microbiomes, which then “program” and regulate the activity of the immune system. It’s also become clear that indiscriminately killing off the body’s bacteria with powerful antibiotics — especially if those drugs are administered when a person is young — can, in some cases, cause or contribute to the development of obesity, asthma, allergies, autoimmune disease, and neurodevelopmental impairments.
On the other hand, science is only just beginning to explore the activities or exposures that support adult microbiome health.
We know that the foods a person eats (or doesn’t eat) can affect the makeup of the gut microbiome. But until very recently, the idea that occupying overly sterile environments could be a problem was not on anyone’s radar. This idea is still controversial, but more and more research suggests that our ongoing interactions with bacteria — for example, when we spend time in nature, with pets, or with other people — are essential to the makeup and health of our microbiomes.
“If you’re in an environment that is ultra clean, I think you can expect that your microbiome will become less rich because you don’t have this constant reinfection through contact,” Lorenzi says. “There have been discussions at NASA about whether it would actually be better to create a greener, more natural environment inside the spacecraft so that the astronauts can be in contact with the normal microorganisms that they interact with and that support immune health.”
In space, this is problematic for a lot of reasons. “One of the goals of space exploration is to identify life on other planets, so it’s important to reduce the risk of contamination as much as possible, and that includes astronauts and their spacecraft,” he explains. “So that’s one of the motivations to keep everything as sterile as possible.”
But here on Earth, Lorenzi’s research — coupled with thousands of recent studies on the human microbiome — suggests that our efforts to scrub, disinfect, and sterilize our bodies, cars, workplaces, and living spaces may be much more likely to deprive us of contact with good bacteria than to spare us interactions with pathogens. This deprivation may be contributing to the steep and ongoing rise of immune-related and noncommunicable diseases, from IBD and other gut conditions to metabolic syndrome, cancer, and autoimmune disease.
Even in this moment, as we’re still struggling to eradicate the coronavirus, the CDC has acknowledged that the odds of picking up the virus by touching a contaminated object or surface is “less than 1 in 10,000,” and that cleaning your hands before you eat would more or less eliminate this already slight risk.
“When I was a kid, people thought that all contact with microbes was bad,” Lorenzi says. “But now we know that actually only a very few microorganisms are able to cause sickness, but many are fundamental for our survival.”