Introduction Microbes and the Intestinal Environment

Microorganisms represent an essential, functioning component of the mammalian intestinal lumen. While the stomach is sparsely populated by acid-tolerant microbes, post-gastric sites support an increasing microbial population density, which in humans can reach concentrations of up to 1011 bacteria g-1 of lumen contents in the large intestine (Salminen et al., 1998). Indeed, the human body contains approximately tenfold as many bacterial cells as somatic cells. Colonization of the human intestinal tract by microorganisms begins peri-natally, when a newborn baby first encounters maternal and environmental microbes during and immediately following delivery. As neonatal development continues, there is a succession of colonization of the infant's developing intestinal tract by major groups of bacteria, which, under normal circumstances, begins to stabilize during weaning (Mitsuoka and Hayakawa, 1973). A stable intestinal microflora is typically attained post-weaning and during early childhood, and forms an essential component of the functioning human body. Perturbations of this resident microflora (for example, by external stressors, dramatic alterations of the diet or antibiotic treatment) can lead to a deterioration of physiological function and decline in health, including poor digestion and nutrient assimilation, immune dysfunctions and susceptibility to infection by diarrhoea-causing pathogens.

The intestinal microflora constitutes a metabolically active microbial environment, dominated by a relatively low diversity of genera, which, in the gut of healthy individuals, exist as part of a stable community (Fuller, 1992). Under normal circumstances, these resident gut bacteria cause neither pathogenesis nor inflammation in the host, but instead contribute to health maintenance, by forming a barrier layer against colonization by pathogens and by aiding in nutrient digestion and assimilation (Salminen et al., 1998). In addition, the resident intestinal microflora plays other important physiological roles in health

© CAB International 2002. Nutrition and Immune Function (eds P.C. Calder, C.J. Field and H.S. Gill)

maintenance: deconjugating potentially damaging oxidative metabolites and toxins in the gut; degrading potentially allergenic food proteins; regulating cholesterol and triglyceride uptake; increasing vitamin biosynthesis; and providing immunosurveillance signals to limit intestinal-tract inflammation. Thus, a stable, properly functioning and active intestinal-tract microflora is essential to the continuance of human health.

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