Abstract
The gastrointestinal tract is colonized with more than 1014 micro-organisms that weigh more than 1 kg. Such microflora are in close contact with the largest mass of lymphoid tissue in the body, the gut-associated lymphoid tissue, and thereby provides the principal driving force in postnatal maturation of the mammalian immune system. Our studies of germ-free animals showed that if such stimuli are not available to the developing immune system during infancy, the maturation of Th 1-dependent immune-deviation mechanisms is inhibited, resulting in a possible lifelong dysregulation of Th 2 response. Recently, several studies showed that psychological stress disrupts the integrity of the intestinal microflora and thereby contributes to the symptomatology of functional gastrointestinal disorders. Furthermore, our recent work demonstrated that such microflora modulate the regulation of hypoth-alamic-pituitary-adrenal axis function in response to stress exposure, which indicates bidirectional communication between the brain and intestinal microflora. Taken together, these results suggest that intestinal microflora and/or their bacterial products, such as lipopolysaccharide and peptidoglycan, stimulate phagocytic cells within the o gut-associated lymphoid tissue, the brain, or elsewhere, and thus influence the function not only of immune system but also of central nervous system.
