The use of probiotics (also referred to as direct-fed microbials) in relation to feed supplements for animals was initiated in approximately 1974, although the suggestion for the use of such bacteria for the human diet dates much earlier. In the early 1900s, Eli Metchnikoff advocated that humans should consume milk fermented with lactobacilli in order to displace the undesirable microorganisms that may occur in the intestinal tract. Since the 1970s many feed supplements have been marketed as sources of probiotics. Unfortunately, in the late 1970s many of these products contained few, if any, viable probiotic bacteria. Thus, many reports in which probiotics were evaluated provided no conclusive evidence about their efficacy. This was in part due to low viability of the microorganism in the products and was further complicated by the fact that the probiotic bacteria, especially the lactobacilli, exhibit host-specificity, which was not considered. For example, one strain of Lactoba-cillus isolated from one animal species is not expected to function well in another animal species.

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