Prebiotics are indigestible carbohydrates. These prebiotics are said to feed/stimulate the growth and activity of beneficial bacteria (probiotics) of the intestinal flora.

Though prebiotics may feed the healthy flora in the gut, some specialists say it feeds the bad bacteria as well. Prebiotics that I hear about the most are FOS (which is often made from sucrose), and Chicholin (made from the chicory root. It's also a form of Inulin). Use with caution if at all.

The theory that prebiotics feed both bacteria might help to explain why some people have bad reactions to foods that are said to help, like garlic. Some people think garlic is a fantastic fungal fighter, while others say it just makes them sick. Garlic is actually a prebiotic. So are onions. I look forward to more conclusive studies being done.