Background: Persistent low-grade immune-inflammatory processes, oxidative and nitrosative stress (O&NS), and hy- pothalamic-pituitary-adrenal axis activation are integral to the pathophysiology of major depressive disorder (MDD). The microbiome, intestinal compositional changes, and re-sultant bacterial translocation add a new element to the bi- directional interactions of the gut-brain axis; new evidence impli...