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Lactic acid bacteria present in the human and animal gut are introduced through fermented milk products, food and feed supplements containing viable bacteria. Lactobacillus species, such as L. acidophilus, L. bulgaricus, L. lactis along with B. bifidum and Streptococcus faecium constitute an integral part of the healthy gastro‐intestinal microecology and are involved in the host metabolism. They impart nutritional and therapeutic benefits to the consumer. The vitamins and enzymes produced by the lactic acid bacteria contribute to host metabolism. The antimicrobial substances produced by these bacteria control the proliferation of undesired pathogens. They also elaborate enzymes that aid host metabolism. This is particularly true in case of lactase nonpersistent individuals. Their anticholesteremic properties assist in lowering serum cholesterol. It has been suggested that the tumor suppression trait of these microbes reduces the incidence of colon cancer.

Nutritional and therapeutic qualities of fermented dairy products are reviewed. Partial hydrolysis of milk constituents (proteins, fats, and lactose) in yogurt, cheese, and other cultured dary foods appears to contribute to their increased digestibility. Lactase and other constituent enzymes of various culturing organisms should contribute to assimilation of lactose by lactose intolerant individuals. Several lactic cultures synthesize certain B-vitamins in fermented dairy products. In contrast, directly acidified dairy products do not exhibit such enhancement in B-vitamins. The hypocholestremic effect of milk is enhanced by fermentation or inclusion of lactic cultures. Lactobacillus acidophilus, Lactobacillus bulgaricus and other lactic organisms produce antimicrobial agents and natural antibiotics. However, production of natural antibacterial substances by different strains of the same species vary widely. These metabolites in cultured dairy products may be responsible for increased shelf life of the foods by inhibiting a wide spectrum of food spoilage organisms. Also, consumption of cultured products containing such natural antibacterial substances may provide the consumer with protection against disease organisms. Unfermented milk containing a specific culture or strain may be consumed to invest organisms for projected beneficial effects.

Consumption of Lactobacillus products and supplements containing viable lactic acid bacteria results in their establishment in the gastrointestinal tract. Their presence in the intestinal tract has been suggested to be prophylactic. They may reduce risk associated with dietary onset of carcinogenesis directly due to the reduction of procarcinogenic substances or indirectly due to the reduction in the level of enzymes that convert procarcinogens to carcinogens. In short-term studies, feeding as well as intraperitoneal administration of lactic acid bacteria suppress tumors implanted in rodents. The suppression is probably mediated by lactic acid bacteria through activation of host defense system. The activation of host defense system has been hypothesized to augment the immune system. Transient increase in the host immune defense system has been observed following feeding of cultured dairy products, but the biological significance of these changes has not yet been elucidated.

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