Probiotics are bacteria that help maintain the natural balance of organisms (microflora) in the intestines . The normal human digestive tract contains about 400 types of probiotic bacteria that reduce the growth of harmful bacteria and promote a healthy digestive system. The largest group of probiotic bacteria in the intestine is lactic acid bacteria, of which Lactobacillus acidophilus, found in yogurt, is the best known. Yeast is also a probiotic substance. Probiotics are mainly used to treat problems in the stomach and intestines, though they also have other uses.