Probiotics: live microorganisms (usually bacteria) that are similar to beneficial microorganisms found in the human gut that are taken as dietary supplements or found in foods. Most probiotics are bacteria similar to those naturally found in the intestine. Common examples are Lactobacillus and Bifidobacterium . They may occur naturally in yogurts and certain fermented foods. Probiotics have been used as treatment for various gastrointestinal conditions including irritable bowel syndrome and traveler's diarrhea.