Astronauts on board the International Space Station (ISS) live and work in a hyper-organized place packed with computers and lab experiments. The quarters look confined and clean.
But on the microscopic level, the ISS, like our homes and workplaces, teems with bacteria.
And in space, microgravity (weightlessness), cosmic radiation, and psychological stress can weaken the immune systems of astronauts and at the same time propel bacteria to become stronger.
A study by German scientists, published in the journal Frontiers in Microbiology, has shown that a silver- and ruthenium-based antimicrobial coating dramatically reduced the number of bacteria on contamination-prone surfaces in the ISS.
The coating, called AGXX and manufactured by the German company Largentec Vertriebs in Berlin, could help protect future astronauts.
Here on Earth, the coating is undergoing tests for possible use in hospitals and medical- and water-system applications.
Elisabeth Grohmann, PhD, is senior author of the study. She’s a professor of microbiology in the department of life sciences and technology at the Beuth University of Applied Sciences in Berlin.
“Spaceflight can turn harmless bacteria into potential pathogens,” she told Healthline. “Just as stress hormones leave astronauts vulnerable to infection, the bacteria they carry become hardier — developing thick protective coatings and resistance to antibiotics — and more vigorous, multiplying and metabolizing faster.”
In addition, the genes responsible for these new traits can be readily shared among different species of bacteria via direct contact or in the “matrix” of slime they secrete, she says.
Grohmann and her colleagues conducted their study from 2013 to 2015.
ISS crew members applied the AGXX on the exterior surface of the restroom door. Performance tests of the coating were made after 6, 12, and 19 months.
“We successfully applied AGXX, a novel antimicrobial material, which can be used as a coating for basically any kind of material, from metals to plastic,” Grohmann said.
“It strongly reduces the growth of bacteria, including many dangerous pathogenic bacteria. The coating kills the bacteria by importing toxic, highly reactive substances (reactive oxygen species) into the bacterial cells. These substances attack the biomolecules in the bacterial membranes, thereby killing the bacteria,” she explained.
After six months, no bacteria were recovered from the coated surfaces of the ISS.
Even at 12 and 19 months, Grohmann says, a total of just 12 bacteria were recovered — a reduction of 80 percent compared with uncoated surfaces.
A regular silver coating tested for comparison had only a slight antimicrobial effect and reduced the number of bacteria by 30 percent.
“With prolonged exposure time a few bacteria escaped the antimicrobial action,” she said. “The antimicrobial test materials are static surfaces, where dead cells, dust particles, and cell debris can accumulate over time and interfere with the direct contact between the antimicrobial surface and the bacteria.”
AGXX contains both silver and ruthenium conditioned by a vitamin derivative. It kills many bacteria as well as certain fungi, yeasts, and viruses, Grohmann says.
The effects are similar to bleach, except the coating is self-regenerating so it never gets used up, she adds.
“But I want to emphasize that the study was initiated not due to health problems of crew members but due to material corrosion on the ISS caused by microbial growth and biofilms on rubber seals, on viewing windows, and on different hardware surface,” Grohmann noted.
David Coil, PhD, a microbiologist and project scientist at the University of California, Davis, also has participated in published studies of bacteria on the ISS.
“In the first study, we took a bunch of bacteria from Earth and compared their growth on the ISS and on Earth. We found that almost every bacteria grew very similarly, with one that grew better in space. We did not look at antibiotic resistance, biofilm formation, or anything like that,” he told Healthline.
“The second was a DNA-sequencing survey of the ISS. We looked at which bacteria were present on surfaces in the ISS. Our major take-home message was that the ISS is dominated by human-associated bacteria, and in fact looks very similar to a home on Earth,” he added.
The antimicrobial coating seems to work well on Earth and the ISS, he says, but the recent study is framed “in more of a scaremongering context than I think is justified,” Coil said.
“There is indeed some work showing that bacteria behave differently on the ISS (biofilm formation, etc.), but virtually none of that work has been translated to an actual increase in virulence or risk,” he said. “Most of the findings, both in this paper and referenced by this paper, lack appropriate context.”
The study authors stated that 60 percent of their bacterial strains had resistance to three or more antibiotics, Coil says.
But that finding doesn’t mean anything without comparison with equivalent strains on Earth, he points out.
“All of the ‘human pathogens’ are from groups that are known to form biofilms, be resistant to antibiotics, and undergo horizontal gene transfer,” Coil said.
“What they really mean is organisms from groups that are known to contain pathogens. Think of E. coli. It can be a pathogen and it can also be an important, beneficial bacteria in humans. I find it very misleading to report only this kind of data from the ISS. It’s easy to draw the inference that these traits are ‘because’ the bacteria were from the ISS,” Coil said.
Coil says the study also doesn’t discuss the idea of neutral and beneficial bacteria.
Is it a good idea to kill everything on board the ISS? “Probably not,” Coil said.
Evidence from hospitals reveal that when medical staff work hard to create a sterile environment, those environments tend to then get colonized with the most resistant and potentially dangerous organisms, he says.
“Yes, astronauts have impacted immune function in space,” Coil said. “And yes, some bacteria behave differently on the ISS. But I don’t think it’s justified to say things like, ‘Spaceflight can turn harmless bacteria into potential pathogens,'” Coil said.
“Sure, she says ‘potential,’ but I still think the average person reading that sentence would take home the idea that spaceflight makes these bugs dangerous. I don’t see the evidence for that,” he continued.
“While I think the data in this paper is fine,” Coil noted, “I think it’s framed within an overly scary-sounding context and might not be the kind of solution we want for space travel anyway.”
Nonetheless, the antimicrobial coating may still have a bright future in space and on Earth.
“AGXX is being tested for future applications, such as the coating of urine catheters. The first successful study has been conducted for several years but has not yet been published,” Grohmann said.
“Other tests involve its use as an antimicrobial coating for wound dressings, as a germ-killing component of unguents and lotions, and of water-filter systems. A future test will look at killing germs in filters for air conditioners,” she added.
Another current test examines the effect of AGXX on bacterial endospores, the most resistant life-form of some bacteria, and on human pathogenic viruses capable of causing disease, she says.
The Institute of Biomedical Problems at the Russian Academy of Science in Moscow just inaugurated a four-month isolation study using the antimicrobial coating in a manned habitat, a pretest and preparatory research for future lunar and Mars expeditions, Grohmann says.
The study is funded by the European Space Agency and NASA.
The goal is to identify which bacteria survive on the antimicrobial-coated areas and to assess the possible health risks for crew members, including antibiotic resistance, toxin production, virulence factors, and biofilm formation.