Urinary but not brain isatin levels are reduced in germ-free rats

Germ-free rats excreted considerably smaller amounts of the monoamine oxidase-inhibiting compound isatin than the substantially larger output by conventional animals of the same strain, although concentrations in brain and other tissues were similar in the two groups. Thus, isatin is likely to be elaborated both endogenously in rat tissues and "exogenously" by flora inhabiting the lumen of the alimentary tract.