Functional profiling of spacecraft cleanroom microbiomes through genome‑wide phenotype predictions

When we send a spacecraft to another world, we carry a hidden cargo: microscopic life. Even in the most sterile assembly rooms, where technicians wear full-body suits and scrub surfaces with extreme care, tiny bacteria and fungi cling to floors, walls, and equipment. For decades, space agencies have worried that if a probe lands on a planet like Mars, these hitchhiking microbes could survive the journey, wake up, and multiply, potentially contaminating the alien environment and confusing our search for native life. This concern is known as forward contamination. The current standard for checking how many microbes are present relies on a simple test: heating a sample to kill off most living things, then seeing which spore-forming bacteria survive to grow on a petri dish. This method works well for finding tough, spore-forming organisms, but it misses a vast number of other microbes that do not make spores yet might still be tough enough to survive the harsh conditions of space and the icy, salty oceans of moons like Europa.

A new study by a team of researchers from Europe and the United Kingdom tackles this blind spot by looking directly at the genetic blueprints of the microbes found in the cleanrooms where the European Space Agency's JUICE mission was prepared. Instead of trying to grow these organisms in a lab, the scientists used powerful computers to read the DNA of the entire microbial community. They built a new kind of digital tool that acts like a fortune teller for bacteria, predicting which ones have the specific genetic traits needed to survive extreme cold, drying out, or high salt levels. By applying this tool to the microbes found on the floors of the launch facility in Kourou, French Guiana, the team discovered that the risk of contamination is more complex than previously thought. They found that some common skin bacteria, which usually seem harmless, carry a surprising combination of survival skills that could allow them to endure the journey to Jupiter's icy moons.

The researchers started by swabbing large areas of the floor in two specific cleanrooms during the assembly of the JUICE spacecraft. They collected enough material to extract DNA and then sequenced it, reading the genetic code of millions of microbial fragments. Because the amount of DNA was so small and the environment so clean, they had to be extremely careful to distinguish real microbes from background noise. Using advanced software, they pieced together 183 partial genomes, which are like jigsaw puzzles where they managed to reconstruct 25 complete enough pictures to study in detail. These reconstructed genomes represented the actual microbes living on the floor, including common residents like Cutibacterium acnes and various types of Staphylococcus, which are typically found on human skin.

Once they had these genetic blueprints, the team applied a new machine learning system they had developed. This system was trained on thousands of known bacteria to recognize specific genetic patterns that indicate a microbe's ability to survive in harsh conditions. The researchers asked the computer to look for six key traits: the ability to live without oxygen, to make its own food from simple chemicals, to survive freezing temperatures, to withstand drying out, to handle high salt concentrations, and to form protective spores. The computer analyzed the 25 reconstructed genomes and found that several of them possessed multiple survival traits at once. For instance, certain strains of Staphylococcus and a bacterium called Romboutsia were predicted to be able to survive without oxygen, resist drying out, tolerate high salt, and in the case of Romboutsia, even form spores.

The study revealed that these microbes were not just dormant passengers; some appeared to be actively growing at the time of sampling. The researchers calculated that 16 of the 25 reconstructed genomes showed signs of active replication, meaning the cells were dividing. While the most abundant microbes on the floor were not necessarily the ones with the most dangerous traits, the ones that did have multiple survival skills were present in significant numbers. The team also compared their findings from the JUICE launch site with a massive collection of 1,868 genomes from other sources, including the International Space Station and various human skin studies. They found that the microbes at the launch site had a unique mix of traits, including a higher-than-expected ability to survive in cold, salty conditions, which are the exact conditions found on Jupiter's icy moons.

One of the most important conclusions of the work is that knowing the name of a microbe is not enough to judge the risk it poses. The study showed that even within the same species of bacteria, different strains can have very different survival abilities. A standard test that only identifies the species might miss a particularly tough strain that has the genetic tools to survive the journey to an icy moon. The researchers argue that we need to move beyond simple counting and naming of microbes and instead focus on their functional capabilities—what they can actually do. Their new approach, which combines deep genetic sequencing with computer predictions, offers a way to see these hidden capabilities.

The team emphasizes that their computer predictions are not a final verdict but a powerful guide. The models they built are good at spotting potential risks, but they are not perfect. Some traits, like the ability to survive extreme drying, were harder to predict accurately than others, such as the ability to form spores. The researchers suggest that these computer predictions should be used to prioritize which microbes need to be tested in the lab. Instead of testing every single microbe found in a cleanroom, scientists can now focus their efforts on the specific strains that the computer flags as having the highest chance of surviving the trip to an alien ocean. This targeted approach could make planetary protection more effective and ensure that we do not accidentally introduce Earth life to worlds where it accidentally proliferates.

The study also highlights the specific challenges of protecting icy worlds like Europa. Unlike Mars, which is dry and cold, these moons have subsurface oceans that are liquid and rich in the chemical ingredients needed for life. If a microbe from Earth can survive the long, cold journey through space and then find a way into that ocean, it could potentially thrive and spread. The researchers found that the microbes at the JUICE launch site included strains that are tolerant to cold and salt, traits that are directly relevant to the environment of Jupiter's moons. This suggests that the current methods of checking for contamination might be underestimating the risk for these specific types of missions.

By bridging the gap between molecular biology and advanced computing, this work provides a new framework for understanding the microbial risks of space exploration. It shows that the microbes living in our cleanrooms are more diverse and adaptable than we realized. The researchers did not find a single "super-bacterium" that would definitely survive, but they did find a community of microbes where several members carry the genetic toolkit for survival. This discovery means that space agencies need to update their safety protocols to look for these functional traits, not just for the presence of spores. The goal is to ensure that when we send our probes to the icy moons of the outer solar system, we are confident that we are not carrying our own microscopic passengers along for the ride.