The condition I know about is TRPV1 related infections. This is the pathology which is researched to be "the" IBS-D. Though, of course IBS-D is a syndrome, which by definition has no cause, but that is what they call it trying to find a single pathology which most represents what the syndrome is...
API Quickstart is fairly cheap source of nitrifying bacteria which is used for aquariums that you can get for as little as $10. The AO Biome Nitrifying bacteria product is $70. The API quickstart has some additional bacteria for the cycle needed for acquariums that we may not need for human skin...