Probiotic

A probiotic is a living microorganism that is eaten, drunk or applied to the body because, in a big enough dose, it benefits health. The definition set by an expert panel of the FAO and WHO in 2001, and kept by later scientific consensus, has three parts: the microbes must be alive when used, they must be given in a sufficient amount, and a benefit must be shown in studies. Most probiotics are lactic acid bacteria, from groups long known as Lactobacillus and from Bifidobacterium, and some are yeasts such as Saccharomyces boulardii. They occur in yogurt with live cultures, kefir and other fermented foods, and in supplements.

Probiotics may work by competing with harmful microbes for space and food, making acids and antimicrobial substances, strengthening the gut lining and nudging the immune system. Effects depend on the strain, so a result for one strain cannot be assumed for another. Some of the better evidence concerns preventing diarrhea linked to antibiotics and protecting premature infants from necrotizing enterocolitis, while many other claims remain unproven. They are usually safe for healthy people, but serious infections have occurred in very ill or premature patients.

Prebiotics are different: they are nondigestible food components, such as certain fibers, that feed helpful microbes already in the gut.