Probiotics: determinants of survival and growth in the gut

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 in tensely 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. Sur vival rates have been estimated at 20-40% for selected strains, the main ob stacles to survival being gastric acidity and the action of bile salts. Alt hough it is believed that the maximum probiotic effect can be achieved if t he 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 Thu s, 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 vit ro and also in the small intestine of infants; however, it is unlikely that they function in the colon. However, lactulose and certain fructose-contai ning compounds, called prebiotics, are not digested in the small intestine but pass into the cecum unchanged, where they are selectively utilized by p robiotics. Beneficial effects may thus accrue from exogenously administered probiotics, often administered with prebiotics, or by endogenous bifidobac teria and lactobacilli, whose metabolic activity and growth may also be enh anced by the administration of prebiotics.