The wall lining of the gastrointestinal tract is constantly in contact with microorganisms or ingested food. About five hundred species of bacteria live in the adult gastrointestinal tract, mainly in the large intestine.

This community of microbes not only live in peaceful coexistence with humans, but also plays an important role in the welfare of the individual. There is a constant and complex interaction between these bacteria, cells in the intestine and immune system.

Probiotics are defined as live microorganisms which, when administered in adequate amounts, confer a beneficial effect on health of people.

The most frequently used probiotics are bacteria of the genus Lactobacillus and Bifidobacterium that are part of normal healthy bowel. Other probiotics include yeasts.

Probiotics show strain-specific differences in their resistance to acid and bile, ability to colonize the gastrointestinal tract and its clinical efficacy.

The effects of probiotics may be due to several mechanisms of action, including the overgrowth of harmful bacteria, blocking junctions of these bacteria to the bowel wall, increased mucus and regulation of intestinal immune response.

Currently, probiotics are widely sold in capsules, powders and functional foods such as fermented milks and yogurts.