Untargeted Metabolomics Identifies Faecal Filtrate-Derived Metabolites That Disrupt Clostridioides difficile Metabolism and Confer Gut Barrier Cytoprotection.

Recurrent Clostridioides difficile infection (rCDI) remains a major therapeutic challenge. Although faecal microbiota transplantation (FMT) is highly effective and thought to restore microbial composition and metabolic function, the mechanisms underlying its success are not fully understood. In particular, the contribution of non-bacterial components such as soluble metabolites remains unclear. Therefore, further investigation is needed to identify the mechanistic drivers of FMT efficacy and clarify how non-bacterial factors contribute to therapeutic outcomes. Here, we applied untargeted three-dimensional Orbitrap secondary ion mass spectrometry (3D OrbiSIMS) to profile faecal metabolic reprogramming in rCDI patients pre- and post-FMT, alongside C. difficile cultures exposed to sterile faecal filtrates. FMT induced extensive metabolic shifts, restoring glyoxylate/dicarboxylate and glycerophosphoinositol pathways and normalising disrupted bile acid and amino acid profiles. Faecal filtrate exposure caused strain-specific metabolic disruption in C. difficile, depleting proline, fumarate and succinate while enriching tryptophan. While multiple metabolite classes were profiled, the most significant functional changes were observed in lipids. Lipidomics identified >3.8-fold enrichment of phosphatidylinositol (PI) species, which localised to bacterial membranes and conferred cytoprotection against C. difficile toxins and other epithelial insults. Spatial metabolomics imaging revealed, for the first time, metabolite compartmentalisation within C. difficile, with proline and succinate broadly distributed across the cell surface and fumarate confined to distinct microdomains, highlighting functional heterogeneity in pathogen metabolism. Collectively, these findings demonstrate that soluble metabolites within faecal filtrates mediate pathogen suppression and epithelial barrier protection, establishing metabolite-driven mechanisms underlying FMT efficacy and identifying PI lipids as candidate post-biotic therapeutics for rCDI.