Major Depressive Disorder

Major depressive disorder (MDD) is a psychiatric condition characterized by persistent depressed mood, anhedonia, and cognitive/somatic symptoms. In the WikiBiome framework, MDD is a paradigm case for the gut brain axis: gut dysbiosis drives systemic inflammation and neuroinflammation through endotoxemia, tryptophan-kynurenine shunting, and loss of neuroactive microbial metabolites (serotonin, gaba).

Microbiome Signature

  • Depleted: dialister, faecalibacterium prausnitzii, coprococcus, roseburia — SCFA and neuroactive metabolite producers [1] [2].
  • Enriched: Enterobacteriaceae, Eggerthella — LPS-producing pro-inflammatory taxa.
  • Metal connection: Shotgun metagenomics in MDD reveals nickel-metal toxicity signatures (NiMeTox) [3].
  • Mediterranean diet: Associated with reduced MDD risk via microbiome modulation [4].

Mechanistic Pathways

  1. Tryptophan shunting: Inflammation-driven IDO activation diverts tryptophan from serotonin synthesis to the neurotoxic kynurenine pathway → quinolinic acid → excitotoxicity.
  2. SCFA depletion: Lost butyrate → impaired BBB integrity → neuroinflammation.
  3. Endotoxemia → neuroinflammation: LPS → TLR4 → IL-6/TNF-alpha → microglia activation.
  4. Vagal signaling: Depleted Lactobacillus/Bifidobacterium → reduced vagal tone → HPA axis dysregulation.

Cross-References

References (7)

  1. Capuco A, Urits I, Hasoon J et al. (2020). Current Perspectives on Gut Microbiome Dysbiosis and Depression. Advances in Therapy. doi:10.1007/s12325-020-01272-7
  2. Eskandar K (2025). The Gut-Brain Axis in Depression, Anxiety, and Schizophrenia: A Scoping Review of Mechanisms, Biomarkers, and Therapeutic Implications. Middle East Current Psychiatry. doi:10.1186/s43045-025-00585-z
  3. Michael Maes, Abbas F. Almulla, Asara Vasupanrajit et al. (2026). Maes 2026 — Functional Shotgun Metagenomic Insights into Gut Microbial Pathway and Enzyme Disruptions Linking Metabolism, Affect, Cognition, and Suicidal Ideation in Major Depressive Disorder. Acta Neuropsychiatrica. doi:10.1017/neu.2026.10056
  4. Adriel Latorre-Pérez, Marta Hernández, Jose Ramón Iglesias et al. (2021). Latorre-Pérez 2021 — The Spanish Gut Microbiome Reveals Links Between Microorganisms and Mediterranean Diet. Scientific Reports. doi:10.1038/s41598-021-01002-1
  5. Siegmann EM, Muller HHO, Luecke C et al. (2020). Graves' disease as a driver of depression: a mechanistic insight. Frontiers in Endocrinology. doi:10.3389/fendo.2020.00530
  6. Zhang Ruohan, Wang Ruting, Wu Hongxi et al. (2025). Zhang 2025 — Gut Microbiota as a Novel Target for Treating Anxiety and Depression: From Mechanisms to Multimodal Interventions. Frontiers in Microbiology. doi:10.3389/fmicb.2025.1664800
  7. Yimin Han, Boya Wang, Han Gao et al. (2022). Han et al. 2022 — Vagus Nerve and Underlying Impact on the Gut Microbiota-Brain Axis in Behavior and Neurodegenerative Diseases. Journal of Inflammation Research. doi:10.2147/JIR.S384949

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