Among the most predominant microbes in the human intestinal tract are the Gram-positive lactic acid-producing genera Lactobacillus and Bifidobacterium. Lactobacilli and bifidobacteria are also common fermentative microbes in yoghurt, cheese and soured vegetable foods (such as sauerkraut and suguki). The majority of fermentative microorganisms present in such foodstuffs are susceptible to low stomach pH and bile-salt secretions and cannot survive gastric processing. However, following oral delivery, a few strains are able to survive gastric transit and can persist in the intestinal lumen. These strains are thus able to transiently colonize the gut by integrating into the existing microflora, and are termed 'probiotics'.
Probiotics can be defined as dietary supplements containing living microbes that are able to persist in (or transiently colonize) the human intestinal tract and impart a beneficial influence on host physiology, such that this effect is able to improve health. This process is particularly important at times when the normal indigenous microflora has been perturbed: at this point, exoge-nously supplied probiotics of a defined species/strain are able to temporarily colonize the intestinal tract and stabilize the microfloral composition, thus restoring the vital physiological functioning of the microbial community. Thus, the use of probiotics in health improvement relies on the principle that exogenous microbes (from food sources) augment the beneficial physiological effects of the normal (indigenous) gut microflora.
Among the many purported physiological influences of probiotic microorganisms, a large proportion of research attention over the last decade has focused on the interaction of probiotics with the immune system (Salminen et al., 1998). It is evident that several probiotic strains of lactic acid bacteria (LAB) are able to influence the immune system and, in many cases, this effect has been linked to a measurable improvement in health. The immune system comprises innate and adaptive components, and these play vital interacting roles in health maintenance, in both regulating and stimulating the body's responses.
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