Intestinal colonization and programming of the intestinal immune response.

Initial bacterial colonization of the gut is a vital component of the development of the gastrointestinal tract, particularly mucosal immune protection, during the neonatal period. Newborn infants in their protected intrauterine environment are suddenly thrust into a highly contaminated extrauterine state. Although mucosal host defenses have developed in utero during fetal maturation because of the stimulation of ingested trophic factors in amniotic fluid, actual active protection only occurs when colonizing bacteria stimulate the gut mucosal barrier. Colonization evolves over a period of about 1 year and is dependent on the mode of delivery, use of perinatal antibiotics, age at birth, and infant feeding. A fully colonized gut consists of 10(14) bacteria, establishes a symbiotic relationship with the host and insures normal development and immune homeostasis. Colonizing bacteria can also affect the epithelial mucosal barrier and the innate and adaptive immune systems. Disruption of normal colonization, dysbiosis, is associated with increased expression of disease. Evidence exists that the use of probiotics with dysbiosis may prevent disease expression.