Bifidobacteria are typically found in the alimentary tracts of human infants and adults. Many factors control the number and the composition of microbial populations in different regions of the gastrointestinal tract. Ways of restoring or maintaining a proper microbial balance in the gastrointestinal tract have been extensively investigated (see Bifidobacterium as health-promoting agents). Bifidobacteria coexist with a large variety of bacteria, most of which are obligate anaerobes. Components of this microbiota are different in the different areas of the gastrointestinal tract (Moore et al., 1969).