Next, Finlay’s group used stool samples from the asthma-prone 3-month-olds to colonize the guts of mice that had been raised in a bacteria-free environment. The animals went on to develop inflamed lungs indicative of asthma. But if the researchers added a mixture of the four missing microbes to the mice’s digestive tracts along with the feces, the mice no longer had a heightened risk of developing asthma, the scientists report online today in Science Translational Medicine.