Metabolism of saikosaponins by human intestinal bacteria is discussed in view of the results obtained during the last years. Saikosaponins (SAPs) were converted to prosaikogenins and saikogenins in order by a human fecal suspension and by the bacterial strains, Eubacterium sp. A-44, Bifidobacterium sp. Saiko-1 and Saiko-2, isolated from human feces. From E. sp. A-44, two glycosidases responsible for the stepwise hydrolysis of SAPs were isolated and characterized as saikosaponin-hydrolyzing β-D-glucosidase and prosaikogenin-hydrolyzing β-D-fucosidase. After the oral administration of saikosaponin b_1 (Sb_1) to germ-free rats, no metabolites were detected in their plasma, cecal contents and cumulative feces. However, considerable amounts of prosaikogenin A and saikogenin A were detected when Sb_1 was orally given to conventional or the E. sp. A-44-infected gnotobiote rats. Fecal and cecal activities of hydrolyzing saikosaponins were found in the gnotobiote and conventional rats. These findings provided direct evidence that saikosaponins-hydrolyzing bacteria are necessary for the appearance of their biologically active metabolites in the systemic circulation
