It might be that they have to hit a certain critical mass to become self-sustaining. Or they require a particular environment to thrive, which exists in the don

It might be that they have to hit a certain critical mass to become self-sustaining. Or they require a particular environment to thrive, which exists in the donor but not in the donee. It's such a new field, at least for prescriptive medicine, that it'll probably be a while before we get a good base of understand of what lives in our gut, what it does, and how to keep it healthy.