Not my favorite subject, but whatcha gonna poo? Er, do?
First thought, Big Pharma is not going to like this. It is totally out of their control. Expect scare tactics. And there will be no research to speak of.
That means it is up to the patients to test thngs and to report back to the community. The obvious method is the enema. Probably the best donor is a healthy spouse. Another alternative is to figure out a simple way to culture it in yogurt or similar.
Maybe something missing is replaced. Or something unwanted is overcome and eliminated or controlled.
Given a close tie to the immune system, how far could this go? Is there a bug for every disease? Do we end up with a donor system ala blood types? Bacteria are extremely adaptable and rearrange themselves in minutes. Then they tell the others how to do it too. They need us to survive, so it makes perfect sense that they should look after us. What if history puts antibiotics on display with blood letting?
Don't be shy now.
Another thing to be investigated are things like Rid-X septic tank treatments
While obviously not intended for human use it looks possible.