There is a two-way link between gut bacteria and immunity. Immunity often forces composition of gut bacteria regardless of supplementation. Pro/pre-biotics often have no effect, we are still missing something.
There is pretty minimal labeling of probiotic capsules in the grocery store (grr, Orrin Hatch) but then again nobody really knows anyway. I had my gut cleaned out due to a heavy course of antibiotics last summer and just repopulated it by kissing my partner and eating unpasteurized joghurt. Not especially scientific either, but it worked.