Probiotics in pediatrics.

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At birth, the gastrointestinal tract is sterile. Within hours, bacteria ingested during the birthing process rapidly colonize the gut. The gastrointestinal tract soon contains about 10 times as many bacteria as there are cells in the body. Hundreds of species are present, many of which are unculturable and remain unidentified. It is these bacteria that are responsible for priming the gastrointestinal immune system. Studies in germ-free mice have proven that without these bacteria, the systemic immune system will not function normally.1 After this initial colonization, a person’s individual gut flora remain remarkably constant throughout life. The gut immune system learns to recognize and tolerate those bacterial species acquired during early infancy. Consequently, it is very difficult to permanently change gastrointestinal flora after this time. As one would therefore expect, each person’s bacterial flora are remarkably similar to that of his/her mother, as they were initially ingested from the mother’s vagina at birth. The vagina is typically colonized with normal fecal flora.2 Most gastrointestinal organisms are relatively benign. Some are potentially more pathogenic; however, many are actually beneficial. It is these beneficial organisms that have attracted attention as possible probiotics. Probiotics are defined as live microorganisms that, when ingested, produce some therapeutic or preventive health benefit.3 Because of the difficulty in permanently changing intestinal flora, successful colonization with a probiotic is usually transient. In fact, many so-called probiotics fail to colonize even transiently, as the gastrointestinal tract has many defenses that inhibit colonization. These include gastric acid, duodenal bile, mucin, and the gut immune system.4 Successful probiotics are capable of resisting these insults, and transiently take up …