A recent study review found that 12 out of 16 probiotic studies saw better outcomes when using multiple probiotic strains instead of just a single strain. These studies covered many different topics such as inflammatory bowel disorder, Helicobacter pylori, gut function & others.
I don’t find this terribly surprising as our digestive tract is teaming with all sorts bacterial flora. There is no jack of all trades probiotic that does everything for the digestive tract. There are also many different sub-strains of probiotics out there. If you go to the store you’ll probably see a lot of bottles labeled as being L. Acidophilus, however each company breeds it’s own strain(unless the companies outsourced from same lab) of L. Acidophilus so you might even get slightly different results depending on which brand you choose. There also may be a host of probiotic sub-strains we haven’t discovered yet. This is why something like fecal transplantation is interesting because it takes a working bacterial ecosystem and transplants it into someone who does not have one. It’s hard to slowly rebuild a healthy bacterial ecosystem with only a handful of bacteria, especially when we do not know which bacteria are most effective for each person & we may even be missing some.