Cosmic radiation, microgravity: just two of the things astronauts are exposed to in their work that most of us never worry about here on Earth. But one thing astronauts do not encounter, if at all possible, is a great variety of bacteria. During both training exercises in confined environments and actual spaceflight, technical teams go to great lengths to make sure the astronauts’ environments are impeccably clean—sterilizing the food they eat and the bedding they use, treating their water, and even filtering the air they breathe.

Avoiding every last microbe is impossible, of course, even in the highly controlled environment of a spaceship or a training facility. But on the whole, the environment of an astronaut is one of decidedly low microbial diversity. Astronauts’ bodies are not bombarded with the vast number of foreign substances that would be part of a normal Earthling’s day.

Scientists have seen this anti-microbial vigilance as necessary to protect astronauts’ health, since it’s well known that the job goes hand in hand with reduced immune functioning and vulnerability to pathogens. That is, scientists have found that viruses like varicella zoster virus and Epstein-Bar virus—which the body normally harbors in inactive form because the immune system keeps them in check—can be reactivated in astronauts during space missions, indicating faltering immunity. Even rodents in space show immune system impairments: in one study, mice aboard the International Space Station showed changes in specific immune system parameters over a 91-day period compared to mice kept on the ground. So it seems the emphasis on keeping sickness-causing bacteria out of the environment is warranted.

Scientists are wondering whether astronauts could take a cocktail of probiotics that could help make up for some of the microbes they lack, to support immune function

The relatively recent study of astronauts’ gut microbiota, however, has highlighted a possible problem with these sterile environments. The drive to eliminate all microbes could paradoxically be putting astronauts’ health further at risk—some scientists believe these environments may not be adequately supporting the gut microbiota and is role in maintaining a steady state of health. Of particular note is the crew’s subsistence on preprocessed foods that are free of commensal bacterial species that could “challenge” the gut microbiota.

The immune system constantly monitors its immediate environment, including harmless microorganisms in the gut microbial community, in order to stay ‘alert’ and flexible to a changing environment that contains possible disease-causing organisms. In a project called ICELAND, investigators including Paul Enck, Joël Doré, and John Penders (among others) are studying the crew at the Concordia Antarctic Station (a remote human outpost used to study extreme conditions like space) to assess whether the crew’s extended stay in a confined, sterile environment will dysregulate their immune systems and lead to poorer health because of a low-diversity, “impoverished” gut microbiota. They plan to study the number of total bacteria in the gut, the particular bacteria present, and overall diversity, as well as immune reactivity, repertoire, and memory (for example, the decline of vaccine memory) as a result of a microbially-minimized environment. Future studies may continue in this vein and study the same parameters in astronauts on actual space missions.

If the results in astronauts turn out to support this idea, they would provide cosmic proof for a more down-to-Earth theory: the hygiene hypothesis. This well-known hypothesis cites a lack of exposure to microorganisms, attributable in part to “higher standards of personal cleanliness”, as responsible for the increased incidence of immune-mediated diseases like asthma and allergies. A growing body of evidence supports this idea, and indeed, environments with varying collections of microbes can be studied on Earth (such as a recent comparison of Amish and Hutterite homes and asthma/allergy risk among their children). But none are quite as sterile as the environments encountered by space-bound individuals.

Scientists also wonder whether astronauts could take a specially-designed cocktail of probiotics that could help make up for some of the microbes they lacked, thereby supporting immune function. If this were successful it could add a new dimension to the hygiene hypothesis: the current model mostly looks at how immunity and immune-related diseases play out over a person’s lifetime, but a successful intervention in space could show the potential to modulate immunity, and perhaps disease risk, over a shorter period of reduced microbial exposure. As far-out as it might seem, the analysis of a few brave astronauts could be all it takes to set off future discoveries that help the hundreds of millions on this planet afflicted with asthma or allergies.