Discovery of Human Gut Bacteria Responsible for Biotransformation and Structural Diversification of Oat Steroidal Saponins

Abstract Oat steroidal saponins undergo biotransformation by the gut microbiota, yet the bacteria involved and the structural diversity of the resulting metabolites remain incompletely characterized. Incubation of avenacoside B (AVE-B), a major oat steroidal saponin, with fecal microbiota from six human donors revealed marked interindividual variation in avenacoside E (AVE-E) production. Five bacterial species capable of AVE-B metabolism were isolated, with Bacteroides uniformis SS-F-0001 exhibiting the broadest metabolic profile. Quantitative PCR and carbohydrate-active enzyme profiling suggested that abundance alone did not explain conversion capacity. Comparative 1D and 2D NMR analysis of isolated AVE-E and its isomer revealed diagnostic C-22 and C-25 chemical-shift differences and H2-26 → C-22 HMBC correlations, supporting an isonuatigenin-type isomer with a six-membered F ring. These findings identify bacterial contributors to oat saponin metabolism and reveal structural diversity involving both deglycosylation and aglycone ring rearrangement, providing a basis for investigating the mechanisms and biological consequences of microbial saponin metabolism.