One of the most important ways in which a probiotic organism may exert a beneficial effect on its host is to modify metabolic processes, particularly those occurring in the gut. Such a beneficial effect could be achieved in theory by a variety of mechanisms:
By suppressing reactions which result in the generation of toxic or carcinogenic metabolites.
By stimulating enzymic reactions involved in detoxification of potentially toxic substances, either ingested or formed endogenously.
By stimulating mammalian enzymes involved in the digestion of complex nutrients, or when such enzymes are absent (due to genetics or disease) providing a bacterial source of these enzymes.
By synthesizing vitamins and other essential nutrients not provided in sufficient quantities in the diet...
Book: Fuller, R. (Ed.). (1992). Probiotics, The scientific basis. London: Chapman & Hall.