OpenBiome is similar in some ways to a blood bank, which also collects, screens, and distributes biological material. Early on, though, it became clear that the group faced different challenges in recruiting donors and testing samples. For one thing, few applicants qualified: Anyone who had had a gastrointestinal disease, recently used antibiotics or traveled to any country with a risk of waterborne illness was rejected, even before their stool was analyzed for pathogens. It was sometimes awkward turning away MIT colleagues who wanted to contribute stool, Smith said, but “it helped to say, ‘hey, I’m not eligible either.’”(Smith’s brother lives in Singapore and he visits there each year.) OpenBiome tests stool for an extensive list of pathogens, including HIV, hepatitis, and syphilis, which can be transmitted through bodily fluid. But they also screen for disorders, from obesity to metabolic syndrome to autoimmune diseases, that are not traditionally viewed as contagious but have been at least correlated with disturbances in gut bacteria. At a blood bank, no one worries that transfusions might make patients obese.

Rather than casting its net wide for donors, OpenBiome cultivates a small group of stalwart contributors. That way, the cost of testing, which can be roughly $1500 per donor, is spread over many patients and treatments. Contributors in this model make a major commitment. They are required to bring in samples within an hour of passage, since the bacterial composition of stool changes rapidly with exposure to air. “We have one guy who brings his samples in on [Boston’s T train],” Smith said. But for the most part, donors are students who live or work close by. When the project moved from Alm’s lab at MIT, this spring, to a new location near Tufts University, OpenBiome sought out a new cohort of dedicated graduate students. Their constancy also means the group can reassure doctors that the individual whose stool they are receiving has already cured numerous cases of recurrent C. diff—and has not caused new infections in any recipients.