A growing number of studies have reported changes in the microbiota among patients with Parkinson's disease (PD), Alzheimer's disease (AD), ALS, and Huntington's disease (HD), and although the exact mechanisms underlying these phenomena remain unclear, there is a growing body of evidence to support the concept of a “gut microbiota–brain axis”.

What species might be useful and how to reduce amounts of "bad" species? How could it be administered?