When a new crew member arrives on the International Space Station, the population of humans living in space changes - of course. But so, too, does the population of microbes. Countless types of microorganisms inhabit our bodies, inside and out, and when an astronaut arrives on the station, they bring their specific collection of microbial "hitchhikers" with them.

A new study shows that the microorganisms living on surfaces inside the space station so closely resembled those on an astronaut's skin that scientists could tell when this new crew member arrived and departed, just by looking at the microbes left behind. The findings show how keeping an eye on the tiniest space station residents will be important for protecting the health of astronauts and the spacecraft they occupy. It could even tell us something about relatively closed environments on Earth, like hospitals, where understanding the presence of microbes is key.

Many of the microorganisms living in and around us are harmless or even essential for good health, but some can cause disease or damage structures in built environments. This is why NASA has been following the space station's population of microbes with a series of experiments called Microbial Tracking.

These studies, managed by NASA's Ames Research Center in California's Silicon Valley and funded by NASA's Space Life and Physical Sciences Research and Applications division at NASA Headquarters, allow researchers to learn how the micro-inhabitants of the space station change across locations and over time.

"There's an interplay between the microbial community of the space station and its crew, and understanding the details is important for preventing complications for health or for spacecraft on long-term human space missions," said Crystal Jaing, a biologist at Lawrence Livermore National Laboratory in Livermore, California, and principal investigator of the Microbial Tracking-2 study. The results of MT-2 appeared April 29 in the journal PLOS One.

Sampling the Body and the Space Station
One astronaut took part in this research by providing personal microbiome samples before, during and after the spaceflight. A microbiome is a community of different microbes living together. Using a polyester swab, the individual swabbed areas of the skin, ear, mouth, nostril and saliva.

To understand if the crew microbiome interacted with the space station environment, samples were also taken from eight different locations aboard the space station, such as the dining table, the toilet and the crew quarters. These were collected during the crew member's own flight and the one following their departure.

Back on Earth, the samples were processed at NASA's Jet Propulsion Laboratory in Pasadena, California, before analysis at LLNL to compare the microbiome of the space station environment and the microbiome of the astronaut. For the first time in this type of study, the researchers used a sophisticated technique, called shotgun metagenomic sequencing, to explore every bit of DNA found in the samples.

A Microbial Calendar
Of all the body areas sampled, the populations in the environmental surface samples most closely resembled those found on the skin. In all, the astronaut's microbiome contributed to 55% of the surface microbiome found during the individual's flight, the data showed. This person's microbiome also lingered; it appeared - to a lesser degree - in the surface samples taken four months after that crew member left the space station.

"From the microbe data alone, we could tell when the new person arrived and departed," said David J. Smith, a research scientist at Ames and co-author of the study. "We're used to measuring the passage of time with calendars, but the microbiome transitions essentially tell the same story in this study."

The astronaut's saliva samples turned up other interesting results. NASA has used saliva before to study the immune system and health conditions in astronauts, but this study is the first to use metagenomic sequencing to look in depth at the changes to the saliva microbiome due to spaceflight.

The diversity of species found there decreased in space and rebounded after the person's return to Earth. Some of the species affected are considered potentially disease-causing, and the researchers think saliva samples could become a useful way to monitor crew health.

Indoor Microbes and Human Health on Earth
More data collected from additional crew members will help confirm the trends seen in this study, but the Microbial Tracking research already shows the importance of studying and monitoring microorganisms aboard the space station.

Future studies could dig further into the genetic material of the microbiome to understand which microbial genes most influence the relationship between crew and the microbes around them - and how this could affect their health. Knowing which groups of microorganisms are more or less abundant at certain times or in certain places could one day form the basis for tests able to predict health problems and head them off.

Astronaut health is currently protected through routine monitoring of microbes on the space station, along with good astronaut nutrition and appropriate exercise as well as good personal and space station sanitation processes.

The International Space Station provides a unique environment for studying these topics and could shed light on other contexts where indoor spaces and human health overlap. As an orbiting "building" in space, the space station is perfectly suited to studying the arrival, circulation and transmission of microorganisms.

So, understanding better the interactions between astronauts and microbes could even benefit people in relatively closed habitats on Earth - whether in our homes or hospitals, or aboard aircraft, subways or even submarines.

Although this study used samples returned from space, NASA has the ability to identify microbes in real time aboard the space station and is planning real-time microbial monitoring on future spacecraft as well.

+ The Microbial Tracking-2 findings are further confirmed by the work done on the Microbiome investigation. That study's findings were published in 2019 and described in a related article.

Related Links
Microbial Tracking at ISS
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