Exploring the In Vitro Effect of Psilocybin on the Composition and Metabolic Activity of Gut Microbiota in Individuals with Severe Anxiety

Background/Objectives: This study investigated the effects of a single dose of psilocybin-containing Psilocybe cubensis extract on gut microbiota composition, microbial metabolism, and intestinal immune-related responses. Methods: A single dose of P. cubensis extract (35 mg/L) was subjected to colonic fermentation using the SHIME® model inoculated with fecal microbiota from individuals with severe anxiety. Psilocybin and psilocin concentrations, microbial metabolites, gut microbiota composition, intestinal barrier integrity, and cytokine responses were evaluated. Results: During colonic fermentation, the concentrations of both psilocybin and psilocin decreased significantly, suggesting their metabolism or degradation within the gut microbial environment. Treatment reduced ammonia production by 19–25% and decreased gamma-aminobutyric acid (GABA) levels. By contrast, the concentrations of the short-chain fatty acids acetic, propionic, and butyric increased by 2.26-, 2.69-, and 2.21-fold, respectively, after 20 h of fermentation. Microbial profiling revealed marked increases in the relative abundances of Bifidobacterium and Lactobacillus after 20 h and 15 days, together with a slight reduction in α-diversity. Fermentation supernatants also altered cytokine profiles and influenced intestinal barrier-related responses, although these effects reflected a complex immunomodulatory pattern rather than a uniform anti-inflammatory response. Conclusions: These findings indicate that psilocybin-containing P. cubensis extract can modulate gut microbiota composition and metabolic activity under in vitro colonic conditions. The observed changes in microbial metabolites, bacterial taxa, and host-related responses support the hypothesis that gut-microbiota-mediated mechanisms may contribute to the broader biological effects of psilocybin. However, these in vitro findings do not establish clinical benefits for intestinal or mental health, and further validation in animal models and well-controlled clinical studies is required.