One of the most studied probiotics there is, S boulardii is actually a yeast. It is not digested nor absorbed in the gut and it does not exert its effect systemically. It does not colonize, and when you stop giving it to your dog, it will clear the intestines in about 36 hours. Instead, S boulardii acts locally in the lumen of the gut. During its passage through the intestine, S boulardii mimics the physiological effects of healthy digestive flora, stimulating proper immune response and reducing inflammation. It has direct anti-inflammatory activity, and releases a protein that attracts a number of pathogenic bacteria to bind to it instead of the intestines (it is used in many hospitals around the world for antibiotic-resistant C. difficile). S boulardii is typically so effective against so many gut pathogens, it is the only probiotic approved for medical claim, treating "Travelers Diarrhea." The research supporting its role and importance in IBD is presented here.