Even the hostile conditions of space are no match for Earth's extremophile microbes. Should we be worried?
Deep within the coal beds off the coast of Japan, methane-producing bacteria populate the oxygen-starved environment under the sea floor, far from their terrestrial origins. In Lake Tyrell in northwest Victoria, Australia, single-celled organisms flourish in conditions so saline that all other life is extinguished, leaving only a few hardy salt scrubs at the water’s edge. Above Earth’s atmosphere, hurtling through space at over 27,000 kilometers per hour, a hypervirulent strain of Staphylococcus epidermidis creeps steadily, intractably, across the surfaces of the International Space Station.
If this sounds like the opening to a dystopian science fiction story, don’t panic. Organisms known as ‘extremophiles’ have long been studied for their ability to adapt to extreme environments. In many cases, this has led to therapeutic and technological breakthroughs, including the production of biofuels and temperature-resistant compounds used in a variety of industries.
Space is a uniquely hostile environment that subjects life forms to a combination of microgravity, cosmic and solar radiation, high vacuum and extremes of temperature, all without the protective shield of Earth’s atmosphere. Fortunately, space travel presents a rare opportunity to study the effects of these harmful environmental stresses on organismal biology. Over the last few decades, researchers have discovered that certain types of microorganisms not only survive but also thrive in space. The biomedical impact and biological mechanisms of these tenacious microbes are currently a subject of great interest to researchers at the National Aeronautics and Space Administration (NASA).
Why should we care about microbes in space? After all, humans and microbes coexist relatively peacefully on Earth. Unfortunately, beyond Earth’s atmosphere the microbial world becomes dangerously unbalanced.
The study of microorganisms in space dates back to the early 1960s. Poor sterilisation techniques in the early days of space travel resulted in large unwanted populations of bacteria and fungi swiftly multiplying aboard spacecraft. This struggle between man and microbe has continued unabated throughout the history of spaceflight. Despite strict decontamination procedures, spacecraft such as the International Space Station (ISS) retain a lingering microbial fingerprint, which continues to grow as the astronauts on board breathe, sweat and shed cells within their enclosed environment.
In a study by researchers at NASA’s Jet Propulsion Laboratory, several species of Actinobacteria were found at high levels in the ISS. Previous studies of the ISS have also shown the presence of opportunistic pathogens including Staphylococcus and Aspergillus. Though many of these strains generally do not cause adverse effects in humans, they can become dangerous in patients with compromised immune systems, and certain species of Aspergillus can produce carcinogenic toxins. Scientists are further concerned with biodegradation of essential equipment, as certain microorganisms eat plastic polymers and even metals, and accumulation of these may result in damage to structural materials.
A major issue is the formation of biofilms, slimy clumps of bacteria best known for wreaking havoc in hospitals by growing inside wounds or spreading across the surfaces of medical devices such as catheters. These colonies of bacteria are notoriously resistant to antibiotics and, once formed, incredibly difficult to remove. This becomes an even bigger concern when you consider recent studies showing that bacteria become more dangerous during spaceflight, with increased virulence in Salmonella typhimurium and enhanced biofilm formation in Pseudomonas aeruginosa.
As with extremophiles found on Earth, the unusual environmental conditions of space — such as cosmic radiation and microgravity — result in adaptations in the biological function of both harmless and disease-causing microorganisms. Studies on bacteria performed under simulated spaceflight conditions revealed changes in gene expression, stress response, growth rate and drug resistance. What these effects might mean for crewmembers aboard the ISS is still unknown, but the fear that these mutations might give rise to dangerous, drug-resistant infections has NASA researchers looking for answers.
A key realisation is that the microbes in the surrounding environment are not alone in their ability to impact human health. Microscopic hitchhikers also exist within us and are collectively known as the microbiome. Over the last decade, research into the microbiome has gained popularity, as studies have linked microbial composition within humans to a variety of medical issues, from obesity to colon cancer to autoimmune disease. Space flight has been known to reduce immune function in astronauts, particularly with regards to the suppression of viral infections, suggesting that the delicate balance of microbes within the body is disrupted by exposure to space. Changes in the astronaut microbiome might point towards potential health issues that could arise during space exploration.
A comparative study completed last year examined the biology of a pair of identical twins, one of whom lived on the ISS for nearly one year while the other remained on Earth. Early data made available to the public in January have pointed to some changes that occur in the microbiome after living in space. The ratios of two bacterial groups normally dominant in the gut, Firmicutes and Bacteroidetes, were shifted in the twin living on the ISS, but these changes were temporary and returned to normal after landing on Earth.
A separate microbiome study performed by a team at the J. Craig Venter Institute will analyse changes in the microbial profiles of crewmembers’ saliva, blood, skin, sweat and gastrointestinal tract. The aim is to not only identify changes in the human microbiome during spaceflight, but also characterise the interaction between humans and their surrounding environment as they go about their mission. According to NASA, this experiment will allow researchers to evaluate the effect on human health.
In the meantime, should we be worried about contaminating the Earth with so-called ‘space germs’? For the time being, probably not. Microbial mutations caused by short-term space flight seem to be reversible, but we don’t yet know about the effects of long-term space flight. Furthermore, concerns about ‘back-contamination’ extending back to the 1967 Outer Space Treaty have put firmly into place heavy containment and decontamination protocols for all returned samples and equipment involved in space exploration, including the crews themselves.
What about the other way around? In the search for life on Mars, it’s highly likely that we have already contaminated the surface of Mars with Earth organisms. Even after extensive decontamination protocols, researchers at the Jet Propulsion Laboratory found that the surfaces of the Curiosity rover still carried several species of terrestrial bacteria. We don’t yet know whether these bacteria can survive interplanetary flight or the extreme conditions on the surface of Mars, but it’s enough evidence to prevent the Curiosity rover from going anywhere near Martian water for fear of contaminating the source.
It seems presumptuous to be considering planetary conservation, or lack thereof, before having even taken our first steps on another planet. Nevertheless, it’s unclear whether ‘forward contamination’ is unavoidable, or even undesirable. ‘Terraforming’ seems like a cheesy science fiction trope, but may be a viable strategy for advancing space exploration, according to researchers at NASA’s Marshall Space Flight Center.
Extremophiles still exist in pockets of the Earth and are the foundation for the ‘panspermia’ hypothesis, which purports that microbes can be transported in fragments of rock through space. After arriving in their new home, they transform themselves and their environment, making it habitable for new life. At the very least, extremophiles are much more likely than humans, fragile as we are, to survive the journey, let alone thrive and reproduce after being deposited on another planet.
As for our ever-present hitchhikers, it may be that a changing microbiome will be unavoidable for continued space exploration. As the field of microbiology expands, it becomes increasingly evident that our microbiome is uniquely designed to maintain wellness in a variety of environments. It is well documented that microbiomes differ widely between individuals and cultures, depending on diet, lifestyle, genetics, gender and innumerable other factors.
A complete profile of a ‘healthy’ microbiome remains elusive, and very likely does not exist. Ultimately, what we might consider a normal, or reference, microbial profile is reflective only of a healthy body on Earth. Aeons of homeostatic and selective processes have allowed humans and their pathogens to evolve side by side throughout the course of history, and this is not likely to change any time soon. Rather than attempt to maintain the status quo, it stands to reason that the future of humanity will rely on adaptation to their environment, both inside and out.
Edited by Andrew Katsis