With air filters and weekly wipe-downs and vacuuming, NASA goes to great lengths to keep the International Space Station clean so that astronauts stay healthy. But astronauts still often experience health problems like immune dysfunction, skin rashes and other inflammatory conditions. One reason may be because the ISS might be too clean, a new study suggests.Microbes, tiny living organisms like bacteria and viruses, play an important role in human health. But samples of surfaces in the ISS reflect a striking lack of microbial diversity, Rodolfo Salido Bentez, a bioengineer at University of California, San Diego, and colleagues report February 27 in Cell.Astronauts swabbed surfaces in the kitchen, bathroom, dining space and other areas on the ISS. They then sent the 803 samples to Benitez and colleagues for analysis.The ISS has lower microbial diversity than most buildings on Earth. And nearly all of those microbes come from humans and building materials, while less than 0.3 percent are from natural environmental sources like soil and water. Like most indoor environments on Earth, the vast majority of microbes originate from human skin.Inside and outside the body, microbes compete for resources and space, so maintaining a diverse set keeps any one of them from taking over and causing an health problems. Low microbial diversity in hospitals, for example, leads to a higher risk of infection. Even the microbes in your house can affect your health. One study found that Amish communities have a lower risk of asthma than other communities with similar lifestyles because their household dust contains microbes from farm animals.A bag full of custom-made sampling devices used to collect surface swabs throughout the International Space Station floats in the cupola, an observing area for activities outside the station. NASAIf were outside, were generally exposed to a lot more microbial diversity from touching animals or soil, says Sean Gibbons, a microbiome researcher at the Institute for Systems Biology in Seattle. When were inside, these walls, these surfaces are acting like mirrors. Theyre essentially reflecting back upon us our own microbial diversity.Maintaining a healthy diversity of microbes in confined spaces will be a growing concern as astronauts spend more time in space and new missions begin. Scientists will need to test new ways of adding more good germs to the mix, like bringing animals aboard or stocking the ISS pantry with fermented foods, says Pieter Dorrestein, a chemical biologist at UC San Diego.The reality is that were going to inhabit space at some point, so this work will give us the first insight in terms of the things that we need to add and remove, Dorrestein says. The most important message that we can pass on is how important is to not only look at whats present, but also whats absent.