To understand how probiotics work, it is important to understand a little about the microbiology and physiology of the human gastrointestinal tract.

Human beings, like all animals, play host to many types and high numbers of microbes on our skin, in our mouths, in women's vaginal tracts, and all the way through our gastrointestinal tract. In fact, it has been estimated that there are more microbes associated with the human body (about 1014, or 100,000,000,000,000 bacterial cells) than there are human cells in it (about 1013). In addition to this very large number of bacteria, there also is a very large diversity of bacteria. It has been estimated that more than 400 different species, or types, of bacteria make their homes on humans.

Taking this into consideration, it is not surprising that microbes have been found to play an important role in human health. Most of these bacteria are not harmful, and in fact contribute positively to normal human growth and development. But some of these bacteria can have negative influences. It is therefore important that the balance of microbes be maintained to favor the beneficial bacteria over the potentially harmful ones.