Reduced gut microbiota-derived indole-3-carboxaldehyde and 5-hydroxyindole-3-acetic acid are associated with intestinal barrier dysfunction and inflammation in diarrheic suckling lambs.

BACKGROUND: Diarrhea in suckling lambs is associated with gut microbiota dysbiosis, impaired intestinal barrier function, and enhanced inflammatory responses. However, the specific intestinal microbes and microbial metabolites linked to intestinal homeostasis in suckling lambs remain unclear. RESULTS: In this study, diarrheic lambs showed significantly higher serum diamine oxidase (DAO) activity, D-lactate (D-LA), and pro-inflammatory cytokine levels than healthy lambs. We then compared gut microbial composition and metabolite profiles between healthy and diarrheic suckling lambs. Diarrheal lambs exhibited gut microbiota dysbiosis, characterized by an elevated Bacillota (syn. Firmicutes)/Bacteroidota (syn. Bacteroidetes) ratio, reduced abundance of beneficial commensals including Phocaeicola vulgatus and Bacteroides fragilis, and proliferation of the opportunistic pathogen Clostridium perfringens. Metabolomic analysis showed that, in diarrheic lambs, the tryptophan metabolism pathway was reduced with lower levels of indole-3-carboxaldehyde (IAld) and 5-hydroxyindole-3-acetic acid (5-HIAA) than in healthy lambs. Fecal microbiota transplantation experiments showed that transplantation of microbiota from diarrheic lambs partially recapitulated the donor-associated microbial, metabolic, and inflammatory phenotypes in recipient mice. Finally, functional validation using a dextran sulfate sodium (DSS)-induced colitis model revealed that supplementation with IAld and 5-HIAA significantly alleviated DSS-induced intestinal inflammation and barrier damage, accompanied by downregulated expression of genes related to the Tlr4-Myd88-Nfκb signaling pathway. CONCLUSIONS: Our findings indicate that the gut microbiota-derived tryptophan metabolites IAld and 5-HIAA alleviated inflammation and improved intestinal epithelial barrier function by enhancing tight junction integrity, while also reducing the expression of Tlr4/Myd88/Nfκb pathway-related inflammatory signaling molecules.