I'm reminded for some reason of the fact that they do poop transplants to fill someone else's insides with healthy parasites and the like.
Nitpick, but they are mutualistic symbiotes, not parasites.
Poop transplants to move parasites? Why not just ingest the parasites directly. Seems simpler. And less gross.
Actually, no. Consider this is bacteria we're talking about, not worms, you can't just put them in a glass of water and tell the patient to drink it. Well, maybe you can, I'm just not sure it's a good idea. Anyway, it's simpler to just keep them in the poop.