The supplement aisles of most grocery stores and pharmacies in the United States are bursting with probiotics. These billions of bacteria stuffed into once-a-day capsules claim to provide digestive relief, among other benefits. They're extremely popular, with sales of $36.6 billion in 2015. And for good reason. For many people, various gastrointestinal issues come and go pretty much forever, causing chronic discomfort or worse. But according to gastroenterologists and other scientists, these tiny bugs might not be doing the jobs they claim to do.