Saccharomyces boulardii (S. boulardii) is yeast that works to restore your gut function. While it’s not a bacterium, it acts like one. S. boulardii can be most effective for antibiotic-associated diarrhea. It can also provide relief for traveler’s diarrhea. Studies suggest it may help your intestines fight off unwanted pathogens and ensure they’re absorbing nutrients properly. It should be used with caution in people with inadequate immune systems.