Probiotics are defined by the World Health Organisation (WHO) to be "live microbes which when administered in adequate amounts confer a health benefit to the host". They are frequently, though not necessarily, commensal bacteria. The probiotic generamost often used in small animal practice are Enterococcits, Lactobacillus and Bifidobacterium, which are all normal inhabitants of the colonic flora.The human gastrointestinal tract contains > 100 trillion organisms comprising approximately 500 species, many of which have not been identified. These bacteria are parasites, commensals, or mutualists. Within hours after birth, the gastrointestinal tractis populated with numerous bacterial species, but bifidobacteria predominate in mammals. The change to a more complex diet after weaning allows the microbial population to diversify. Critical differences exist between resident bacteria of humans and other animals. Each organism strain appears to be associated with specific effects on other organisms and on the host, and specific cytokine profiles and biological effects result from these interactions. Gastrointestinal bacteria have coevolved with theirhost and have become highly specialized over time. Composition of fecal microbiota in individual human beings and animals is unique but stable over time. Recent studies show evidence that a healthy composition of the microbiome in the intestine is important for the normal development of the gastro-intestinal immune system, and especially for adequate production of mucosal IgA.
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