Metchnikoff (1908) theorized that Lactobacillus delbrueckii subsp. bulgaricus could grow in the intestinal tract of humans and displace any putrefying bacteria that are present. Displacement of this group of bacteria was thought to reduce production of toxic compounds that adversely affect the human body, thus enabling humans to live longer. Research done since Metchnikoff's period has shown that Lb. delbrueckii subsp. bulgaricus neither survives nor establishes itself in the gastrointestinal tract. However, other species of lactobacilli have been reported to provide some beneficial effects through growth and action in the gastrointestinal tract. This group of bacteria and others are now often referred to as probiotics. Although there are other possibilities, cultures most often mentioned as probiotics for humans include Lb. acidophilus, Lb. casei, and Bifidobacterium species. These species along with Propionibacterium species and Lb. reuteri are the ones most often considered for use as probiotics for livestock. All these species can survive and grow in the intestinal tract, and thus have the potential to provide benefits. Certain yeast cultures also are considered as being probiotics for livestock even though the yeasts are not expected to survive and grow in the gastrointestinal tract.

Bacteria normally used as starter cultures for some fermented milk products, such as Lb. delbrueckii subsp. bulgaricus and Streptococcus thermophilus used to manufacture yogurt, also may provide benefits, but not through the ability to survive and grow in the intestinal tract. Benefits they provide come primarily from serving as a source of enzymes needed to improve digestion of nutrients in the gut. For example, P-galactosidase is needed for hydrolysis of lactose in the small intestine (Gilliland and Kim, 1983).

Whereas some reports indicate that the nutritional value of milk can be improved by certain fermentations (Hargrove and Alford, 1980), this chapter focuses on the potential help or nutritional benefits that result from growth or action of probiotic microorganisms following their ingestion. Several benefits are possible from such microorganisms, including control of intestinal infections, control of serum cholesterol levels, beneficial influences on the immune system, improvement of lactose utilization in persons who are classified as being lactose maldiges-tors, and anticarcinogenic action. Research is continuing in each of these areas to provide definite scientific evidence that could permit specific health claims to be made for dairy products containing one or more of this group of probiotic organisms. Several publications have focused on these in more detail (Gilliland, 1990; Lee and Salminen, 1995; Sissons, 1989). Foods containing such microorganisms may be promoted as functional foods in the future.

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