"Your digestive system is like Casablanca for microorganisms: Some 400 species of bacteria and yeast can be found there. Some are locals, created by your body; others are tourists, just visiting after you ingest them via food. Of these microorganisms, those like salmonella and some species of E. coli are nasty (usually the tourists, naturally). And others like L. casei and L. reuteri can be nice - very nice. "Probiotics are basically any microorganism that, when ingested, may benefit human health,"