Bifidobacteria and lactobacilli are purportedly beneficial to human health and are called probiotics. Their survival during passage through the human gut, when administered in fermented milk products, has been investigated intensely in recent years. Well-controlled, small-scale studies on diarrhea in both adults and infants have shown that probiotics are beneficial and that they survive in sufficient numbers to affect gut microbial metabolism. Survival rates have been estimated at 20-40% for selected strains, the main obstacles to survival being gastric acidity and the action of bile salts. Although it is believed that the maximum probiotic effect can be achieved if the organisms adhere to intestinal mucosal cells, there is no evidence that exogenously administered probiotics do adhere to the mucosal cells. Instead, they seem to pass into the feces without having adhered or multiplied. Thus, to obtain a continuous exogenous probiotic effect, the probiotic culture must be ingested continually. Certain exogenously administered substances enhance the action of both exogenous and endogenous probiotics. Human milk contains many substances that stimulate the growth of bifidobacteria in vitro and also in the small intestine of infants; however, it is unlikely that they function in the colon. However, lactulose and certain fructose-containing compounds, called prebiotics, are not digested in the small intestine but pass into the cecum unchanged, where they are selectively utilized by probiotics. Beneficial effects may thus accrue from exogenously administered probiotics, often administered with prebiotics, or by endogenous bifidobacteria and lactobacilli, whose metabolic activity and growth may also be enhanced by the administration of prebiotics.

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Various biological materials were tested for their growth-promoting activity of several bifidobacterial species in a synthetic medium containing ample sources of inorganic salts, vitamins, nitrogen, and carbon. It was found that only Bifidobacterium adolescentis and B. longum (ATCC 15708) grew optimally or near optimally in the synthetic medium. All the other bifidobacteria tested grew optimally only in the synthetic medium supplemented with a growth promoter. The best growth promoters for all bacteria were bovine casein digest and yeast extract rather than human milk whey. Other growth promoters, including human and bovine milk wheys, hog gastric mucin, and bovine serum albumin digest were effective with some bacterial species but not with others. Bifidobacteria also grew well when the bovine casein digest (20 mg/ml) was used as the nitrogen source. Only the yeast extract was able to improve growth under these circumstances. The nature of these growth factors has not yet been determined.

Abstract:The ability of Bijidobacteriu~ species to survive in the presence of sodium glycocholate, at concentrations ranging from 0 to 3 g liter-*, was tested. Bacterial growth was monitored by measuring the optical density for up to 48 h. All the Bijidobacteriurn strains tested showed growth limitation in the presence of sodium glycocholate. However, after a series of propagations in fresh trypticasepeptone-yeast extract medium devoid of bile salt all bacterial strains retained their activity. It was concluded that Bijidobacteriurn species are able to survive physiological and higher bile salt levels and that bifidobacteria can be considered for use as dietary adjuncts.