Narrative review explores microbiome mechanisms as a promising adjunctive avenue for managing depression, anxiety, bipolar disorder, and schizophrenia.

The human gut microbiome has emerged as a crucial regulator of neurophysiological processes by engaging with the central nervous system (CNS) via the microbiota-gut-brain (MGB) axis. One of the most significant ways gut microorganisms influence brain functions is by altering the levels of neurotransmitters. A significant relationship exists between microbial activity and mood, behavior, and cognition. Gut microorganisms can make or break down bioactive substances like serotonin, dopamine, γ-aminobutyric acid (GABA), glutamate, acetylcholine, and histamine. These microbial modulations influence precursor availability, receptor sensitivity, synaptic signaling dynamics, and neuroimmune modulation, thereby indirectly shaping neurotransmission within central circuits. These neurochemical effects, particularly involving serotonergic, dopaminergic, GABAergic, and glutamatergic pathways, are mediated through microbial metabolites such as short-chain fatty acids (SCFAs), alterations in tryptophan metabolism, immune system activation, vagal nerve transmission, and the control of the hypothalamic–pituitary–adrenal (HPA) axis. Changes in the composition of the microbiome have been frequently linked to mood disorders, such as depression, anxiety, bipolar disorder, and schizophrenia. The current review integrates findings from preclinical and clinical studies on microbiome-related neurotransmitter modulation, emphasizing novel therapeutics such as probiotics, prebiotics, fecal microbiota transplantation, and dietary alterations. Unlike previous reviews that primarily focus on microbiome composition or therapeutic interventions such as probiotics and fecal microbiota transplantation, this review adopts a neurotransmitter-centered framework, integrating microbial regulation of serotonergic, dopaminergic, GABAergic, glutamatergic, cholinergic, and histaminergic systems with the pathophysiology of mood disorders. Connecting microbiota-driven modulation of neurochemistry to mental outcomes offers a promising adjunctive avenue for mood disorder management, pending rigorous mechanistic and clinical validation.