Probiotics are nonpathogenic microorganisms that, when ingested, exen positive influence on the health or physiology of the host." AThey can influence intestinal physiology either directly or indirecuy through modulation of the endog enous ecosystem or immune system Survival rates have been estimated at 20 to 40% for selected strains; the main obstacles to survival are 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 demonstrating 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 daily. Certain exogenously administered substances enhance the actioo of both exogenous and endogenous probiotics. Human milk contains many sub stances that stimulate the growth of bifidobacteria in vitro, especially in the small intestine of infants; however, it is unlikely that these substances funcion in the colon Beneficial effects may thus accrue from exogenously administered probiotics, often administered with prebiotics(nondigestible food ingredients that benefit the bost by selectively stimulating the growth or activity of one or a limited number of bacteria in the colon, or from endogenous bifidobacteria and lactobacilli, whose metabolic activity and growth may also be enhanced by the administration of prebiotics.z? Studies that have shown a sudlcient 1evel of proof to enable probiotics to be used as treatments for gastrointestinal disturbances include: