Gut Bacteria Cross-Feed a Mushroom Antioxidant to Boost Anaerobic Energy Production

The human gut is an oxygen-poor environment where bacteria must rely on alternate strategies to generate energy. One well-known mechanism involves microbial cross-feeding, where some bacteria convert dietary compounds into electron acceptors that neighboring bacteria use for anaerobic respiration. This study reveals a previously unknown cross-feeding pathway centered on ergothioneine (EGT), a sulfur-containing antioxidant found abundantly in mushrooms and fermented foods.

Using LC-MS metabolomics, the researchers identified that Clostridium symbiosum—a common human gut commensal—encodes ergothionase enzymes (CLOSYM_01531 and CLOSYM_03165) that cleave EGT into trimethylamine (TMA) and thiourocanic acid (TUA). Heterologous expression of these enzymes in E. coli confirmed their sufficiency for this catalytic activity. A phylogenomic search of the Genome Taxonomy Database revealed ergothionase homologs are prevalent in Clostridia, Bacilli, and Gammaproteobacteria but absent from Bacteroidia, establishing a taxonomic division between EGT degraders and potential TUA consumers.

Experiments with fecal communities from three mouse vendor strains revealed vendor-specific differences in EGT metabolism: Charles River (CR) communities fully converted EGT to a reduced TUA derivative—3-(2-thione-imidazol-4-yl)-propionic acid—while Taconic communities produced TUA but not the reduced product, and Jackson Laboratory communities showed minimal EGT metabolism. The researchers identified Bacteroides xylanisolvens DSM 18836 and B. ovatus ATCC 8483 as capable of reducing TUA, completing the two-step cross-feeding pathway. TUA reduction by B. xylanisolvens produced a ~4-fold increase in ATP synthesis under anaerobic conditions and enhanced bacterial growth, even in minimal medium lacking other electron acceptors, demonstrating TUA's role as a respiratory electron acceptor.

Co-culture experiments confirmed genuine cross-feeding: C. symbiosum supplied TUA to B. xylanisolvens, which then reduced it to the propionic acid derivative, increasing B. xylanisolvens growth without harming C. symbiosum—a commensalistic interaction. Targeted metabolomics of human fecal samples revealed that TUA and its reduced derivative are selectively produced and consumed in a donor-specific manner, supporting physiological relevance in the human gut. Critically, metagenome analysis showed ergothionase genes are significantly enriched in fecal samples from colorectal cancer (CRC) patients compared to healthy controls, consistent with emerging data linking disrupted intestinal EGT homeostasis to CRC risk.

These findings establish a new paradigm: dietary antioxidants like EGT are not merely passively absorbed but can be actively transformed by gut bacteria into metabolic intermediates that shape energy metabolism across microbial communities. This work has implications for understanding how diet, microbiome composition, and disease risk intersect, and raises the possibility that mushroom or EGT consumption could modulate gut microbial dynamics in disease-relevant ways.