Food (or feed) ingredients that are not digestible by humans (or livestock) that might provide benefit to the consumer by stimulating growth or activity of bacteria in the gastrointestinal tract are considered to be potential prebiotics. The large intestine is the most often considered sight of action for these substances, although they could have some impact on microorganisms in the small intestine.
For the most part, these prebiotic compounds contain oligosaccharides, which are not normally digested in the gastrointestinal tract except by resident bacteria (Fooks et al., 1999). Theoretically, any dietary component reaching the large intestine undigested could be a potential prebiotic. However, oligosac-charides are most often considered and have received most attention as prebiotics. Oligosaccharides that have been considered as prebiotics include fructo-oligosac-charides, gluco-oligosaccharides, galacto-oligosaccharides, transgalacto-oligo-saccharides, isomalto-oligosaccharides, xylo-oligosaccharides, and soybean oligosaccharides. Inulin-type fructo-oligosaccharides have been the ones most investigated as prebiotics. Much of the focus has been on their ability to enhance growth of Bifidobacterium species. These bacteria can hydrolyze such oligosac-charides and use them as an energy source to support their growth. They use them in preference to other complex carbohydrates such as starch. Fermentation of these soluble fibers in the large intestine results in production of short-chain fatty acids (primarily acetic, propionic, and butyric) (Flock and Moussa, 1998). These fatty acids are important to the host in lipid metabolism.
Inulin is extracted from chicory roots with hot water. Partial hydrolysis of this extract yields fructo-oligosaccharides, sometimes referred to as fructans (Roberfroid et al., 1997). These fructans are considered bifidogenic and increase growth of Bifidobacterium species in the intestinal tract, primarily in the large intestine. Galacto-oligosaccharides have a similar effect (Sako et al., 1999). Enhancing growth of this group of beneficial bacteria should improve their ability to exert an antagonistic action toward undesirable intestinal microorganisms such as pathogens. This should result in reduced shedding of intestinal pathogens by both humans and livestock when prebiotics are included in the diet.
Fructo-oligosaccharides in animal diets reportedly decrease the amount of fecal putrefactive compounds released, which implies an alteration in the intestinal microflora (Farnworth, 1993). This may be important in control of odors from livestock wastes.
Prebiotics, particularly oligosaccharides, apparently can be used alone to modify the intestinal flora, particularly in the large intestine. Since prebiotics tend to enhance growth of Bifidobacterium species in the intestine, a product containing a prebiotic and a selected strain of Bifidobacterium species could enhance beneficial effects for the host. This might improve the control of intestinal pathogens or bacteria that create malodors in livestock waste.
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