Initial intestinal colonization in the human infant and immune homeostasis.

The paradigm of disease burden in the developed world has changed drastically in the last few decades from predominately infections to immune-mediated diseases (autoimmunity and allergy) because of alterations in the Western lifestyle (improved sanitation, immunizations, antibiotic usage and altered dietary intake). A diverse balanced microbiota is necessary for the development of an appropriate innate and adaptive immune response. There is strong evidence that disruption of the normal colonization process can lead to alterations in the important symbiotic relationship that is necessary for immune homeostasis. For example, infants born by cesarean section or receiving excessive perinatal antibiotics have inadequate initial colonization and aberrant mucosal immune function. As a result, later in childhood, they express an increased incidence in asthma and autoimmune diseases (e.g. celiac disease). An important component of initial colonization is the infant's diet. Breast milk contains a variety of nondigestible oligosaccharides which function as prebiotics preferentially stimulating proliferation of Bifidobacteria and Lactobacilli, important health-promoting bacteria, and cause fermentation of the oligosaccharides into short-chain fatty acids. In the absence of breastfeeding for the first 6 months of life, formula containing pre- and probiotics may overcome an initial inadequate colonization process and help establish a normal mucosal immune system.