Modulating the gut-joint axis: Bifidobacterium longum subsp. infantis B8762 is associated with selective gut microbial and metabolic alterations in knee osteoarthritis.

Knee osteoarthritis (KOA) is a debilitating degenerative joint disorder characterized by chronic low-grade inflammation and metabolic dysregulation. The gut microbiota has emerged as an important regulator of systemic inflammatory responses. Building upon our previous clinical findings that Bifidobacterium longum subsp. infantis B8762 (B8762) improved clinical symptoms and inflammatory markers in patients with KOA, the present study investigated the associated alterations in the gut microbiome and fecal metabolome. Fecal samples were collected from probiotic (n = 20) and placebo (n = 20) groups at baseline (0 M) and after a 1-month intervention (1 M). Integrated metagenomic and untargeted metabolomic analyses were performed to characterize changes in gut microbial composition, functional potential, and metabolic profiles. Metagenomic reads mapped to the B8762 reference genome showed a greater increase in B8762-associated mapping rates in the probiotic group than in the placebo group, supporting an association between B8762 supplementation and longitudinal changes in the gut microbiome. Longitudinal analysis further demonstrated greater increases in microbial alpha diversity in the probiotic group. Species-level analyses suggested selective alterations in gut microbial composition, with nominally higher relative abundances of Bifidobacterium pseudocatenulatum and Anaerostipes caccae and lower relative abundances of Holdemania filiformis and Lachnospira SGB5077 (nominal P < 0.05). HUMAnN3-based functional profiling identified enrichment of microbial pathways related to carbon utilization and amino acid biosynthesis, including the bifidobacterial shunt and branched-chain amino acid biosynthesis pathways. Untargeted metabolomics identified nominal between-group differences in metabolites primarily related to lipid metabolism, including lower relative abundances of aldosterone and 7α-hydroxy-4-cholesten-3-one in the probiotic group (nominal P < 0.05). Correlation analysis further revealed associations between differential taxa and selected metabolites, suggesting potential links between gut microbial alterations and steroid-related metabolic pathways. Overall, B8762 supplementation was associated with longitudinal changes in B8762-associated genomic signals, gut microbial diversity and composition, microbial functional potential, and fecal metabolic profiles. These findings provide exploratory multi-omics evidence supporting an association between B8762 supplementation and gut microbial-metabolic remodeling in KOA and generate hypotheses for future mechanistic studies of the gut-joint axis.