The ISS.
Image: NASA/Roscosmos

An extensive survey of bacteria and fungi on surfaces inside the International Space Station has revealed an astonishing number of microorganisms living among the astronauts—the health impacts of which aren’t entirely clear.

Since it was first established in 1998, the International Space Station has been visited by hundreds of astronauts (227 to be exact). These trips have invariably introduced an array of microbes to the orbital outpost, as have shipments of cargo. But while astronauts return to Earth, their germs stay behind. New research published today in Microbiome offers the most comprehensive catalogue to date of the bacteria and fungi living on the ISS, detailing the station’s distinctive and ever-changing microbiological profile. This research will now be used by NASA and other space agencies to develop safety measures for the ISS and other long-term space missions.

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As a hermetically sealed environment, the ISS features a distinctive microbiome. Life in space is a bit different for germs than it is on Earth—an environment with negligible gravity, enhanced exposure to radiation, and few competing microorganisms. NASA is very much interested in knowing which germs, and in what quantities, are present on the ISS, and how the station’s microbiome is changing over time.

“Specific microbes in these indoor spaces have been shown to impact human health by influencing our susceptibility to allergies, infectious diseases, or sick building syndrome,” wrote the authors, led by NASA JPL’s Checinska Sielaff and Camilla Urbaniak, in the study. “The influence of the indoor microbiome on human health becomes more important for astronauts during flights due to altered immunity associated with space flight and the lack of sophisticated medical interventions that are available on Earth.”

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Microbes on the ISS are influenced by various factors, including ventilation, humidity, air pressure, and the station’s layout. The quantity and diversity of them is also affected by the number of astronauts on board and the types of activities being performed.

NASA and other space agencies have already tried to monitor the ISS microbial population using traditional culture methods. Trouble is, a significant portion of microbes—between 40 to 46 percent—cannot be cultured (i.e. grown in a petri dish), making detection difficult if not impossible. For the new study, a new sampling and testing protocol was developed utilizing both the traditional culture method and molecular-based techniques, allowing for the most extensive survey of microbes on the ISS to date.

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The eight locations sampled on the ISS.
Image: A. C. Sielaf et al., 2019/Microbiome/NASA

NASA astronauts took swabs using sterile wipes at eight predefined locations on the ISS, on three different occasions during a 14 month period. The locations included both high and low traffic areas, including the viewing window, toilet, exercise platform, stowage rack, dining table, and sleeping quarters. NASA astronaut Terry Virts performed the first two sampling sessions on March 4, 2015 and then three months later on May 15, 2015. NASA astronaut Jeffrey Williams took the third sample a year later on May 6, 2016. The samples were returned to Earth for analysis.

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The ISS may seem like a cold, sterile place in space, but the analysis showed it’s a veritable cornucopia for microbes. The most prolific bacteria, according to culture results, were Staphylococcus (26 percent of total samples), Pantoea (23 percent), Bacillus (11 percent), Staphylococcus aureus (10 percent) and Pantoea conspicua and Pantoea gaviniae (both at 9 percent). The fungal population was primarily comprised of Rhodotorula mucilaginosa.

Most of the microbes on the ISS have a human link. Enterobacter, for example, is associated with the human gastrointestinal tract, and Staphylococcus aureus is often found on human skin. Some of these critters are “opportunistic pathogens,” meaning they’re highly adaptable and can take advantage of unusual conditions to infect a host. Fascinatingly, the microbial profile on the ISS is fairly representative of what we see in other human-built environments on Earth, including gyms and hospitals.

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“Whether these opportunistic bacteria could cause disease in astronauts on the ISS is unknown,” said Sielaff in a statement. “This would depend on a number of factors, including the health status of each individual and how these organisms function while in the space environment. Regardless, the detection of possible disease-causing organisms highlights the importance of further studies to examine how these ISS microbes function in space.”

Results of the analysis showed that fungal communities are stable over time, while the populations of various bacteria tend to ebb and flow, likely the result of different astronauts aboard the ISS, the authors explained.

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Alarmingly, some strains of bacteria could form damaging biological sheets known as biofilms, as the researchers pointed out in the study:

[B]iofilm formation on the ISS could decrease infrastructure stability by causing mechanical blockages, reducing heat transfer efficiency, and inducing microbial influenced corrosion. Some of the microorganisms that were identified on the ISS that have been implicated in microbial-induced corrosion on Earth are Methylobacterium, Sphingomonas, Bacillus, Penicillium, and Aspergillus; however, the role they play in corrosion aboard the ISS remains to be determined. Elucidating the potential ability to form biofilms and the magnitude of actual biofilm formation on ISS surfaces is important during long-term space missions to maintain structural stability of the crew vehicle when routine indoor maintenance cannot be as easily performed.

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Equipped with this knowledge, NASA can now study the potential effects of these germs on human health and the structural integrity of the ISS. These insights will be important not just for the space station, but for long duration flights to Mars and beyond.

As a final note, it’s important to remember that not all microbes are bad. And in fact, we are dependent on a lot of bacteria for our health. The important thing here is in understanding how certain bacteria and fungi might act differently in space conditions, and how those changes could compromise human health. Some bacteria are harder to kill in space, for example. Ridding the ISS of all germs is neither desirable or possible—but this latest study will help to ensure the health and safety of its visitors.

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[Microbiome]