Scientists have suggested making the ISS dirtier.

From the very beginning of space flights, the interiors of spacecraft and orbital stations were sterilized: it was believed that otherwise, the proliferation of microbes in space could pose a threat to humans. One of the issues with this strategy is that it has never been effective enough. The majority of cells in our bodies do not belong to us; they are bacteria and archaea that coexist harmoniously with the human organism.

With a severe shortage of normal "non-human" microbes on orbital stations, some of these "human" bacteria and archaea begin to inhabit the surfaces of the stations and their life-support systems, creating serious microbial contamination of a type not found on Earth. After all, on the planet's surface, "human inhabitants" have no freedom to colonize their surroundings: all the spaces are already occupied by "wild" bacteria, which are better adapted for life outside of us.

However, the authors of a new study in Cell demonstrated that there is another problem with the "keep the station sterile" strategy. They examined 803 samples from the International Space Station (ISS)—100 times more than typically collected for previous studies—and then compared their microbial and chemical diversity with terrestrial samples from cities.

On the ISS, almost all the microbial species found on the walls originated from human skin. The concentration of disinfectants and cleaning agents was uniformly high throughout the station. There were also differences: areas where food was prepared and consumed contained more species capable of living on human food. The "toilet" modules harbored more microbes found in feces and urine (though a similar pattern is observed in our homes).

Similar samples taken on Earth were quite different. The microbial diversity there was significantly higher. Only samples from large hospitals and well-maintained urban apartments were similarly poor in diversity. Additionally, terrestrial samples contained more bacteria living in soil and water. This is quite important because they are often biologically "neutral" to humans, which cannot be said for the microbes that inhabit the human microbiome. While the human host is well-adapted to them, other individuals aboard the ISS often are not.

The researchers suggest deliberately introducing soil and aquatic bacteria from Earth onto the ISS. "There is a significant difference between the exposure to healthy garden soil and being mired in your own waste. Meanwhile, something akin to this occurs when you are in a strictly closed environment with no healthy sources of microbes coming from the outside," noted the authors of the study.

An additional factor: increased microbial diversity reduces the likelihood of asthma and allergic reactions. However, while pointing this out, the authors cautiously noted that microbial diversity does not always reduce allergy frequency in animal experiments, yet they still considered this factor important. In other words, they largely support the "hygiene hypothesis," which suggests that the widespread prevalence of allergies in people with modern lifestyles is a result of excessive sterility in their homes, which are regularly treated with disinfectants. The ISS, the scientists remarked, is an extreme example of such treatment, potentially one of the least safe.

It is likely that the authors' approach has already been tested in practice, albeit during ground-based simulations of space flights. In 1972-1973, the USSR conducted the "BIOS-3" experiment, during which three individuals spent 180 days inside a hermetically sealed and gas-closed module. In terms of module size, duration, and number of participants, the experiment corresponded to a flight to Mars in the "Heavy Interplanetary Spacecraft," which was designed at Korolev's design bureau in the 1960s. During that time, the participants grew wheat and several other crops in a mini-greenhouse, later consuming the products from the cultivated plants.