in the gastrointestinal tract as a modifier of human health.

are common members of the human gut microbiota. Multiple reports have emerged linking the abundance of to health benefits and disease risk in humans and animals. This review highlights findings linking species in the gastrointestinal (GI) tract to health outcomes across a spectrum of disorders, encompassing those that affect the digestive, respiratory, urinary, and central nervous systems. The mechanism through which exerts a beneficial versus a detrimental effect on health is likely dependent on the genetic makeup of the host metabolic capacity and immunomodulatory properties of the strain, the competition or cooperation with other members of the host microbiota, as well as synergy with co-administered therapies.