Saccharomyces boulardii

A probiotic yeast (not bacteria) — which is exactly why it works alongside antibiotics. The most evidence-backed prevention for C. difficile and antibiotic-associated diarrhea.

S. boulardii is a yeast, not a bacterium — which means it's unaffected by antibacterial drugs. Take it simultaneously with antibiotics (unlike bacterial probiotics, which need to be timed apart). It produces antimicrobial substances active against pathogenic bacteria, modifies host immune signalling in the gut, and disrupts C. difficile toxin attachment. Multiple meta-analyses confirm meaningful protection against C. diff and antibiotic-associated diarrhea. Discovered by French microbiologist Henri Boulard, who noticed locals in Southeast Asia chewing lychee skins to treat diarrhea — the active organism turned out to be a yeast on the fruit.