Microbiota–mitochondria crosstalk in the gut–brain axis: a missing link in the etiology and treatment of major depressive disorder

Major depressive disorder (MDD) is increasingly understood as a multifactorial psychiatric disorder involving interacting neural, immune, metabolic, and microbial processes. Within this framework, the microbiota–gut–brain axis and mitochondrial bioenergetics have emerged as potentially intersecting contributors to depressive symptomatology. Preclinical studies suggest that microbial metabolites—especially short-chain fatty acids (SCFAs)—can influence oxidative phosphorylation, redox balance, neuroinflammation, and synaptic plasticity, whereas inflammatory signals such as lipopolysaccharide may disrupt mitochondrial dynamics. However, the strength of evidence is uneven: mechanistic support is strongest in cell and animal models, whereas human data remain heterogeneous and largely associative. This narrative review critically synthesizes current evidence on microbiota–mitochondria crosstalk in MDD, distinguishing established findings from emerging hypotheses. It also examines recent psychobiotic trials, metabolomic and biomarker studies, and microglia–mitochondria mechanisms, and discusses the translational limitations that currently constrain clinical application. Overall, this axis represents a plausible and clinically relevant framework for hypothesis generation and adjunctive intervention development, but it should not yet be regarded as a fully validated causal pathway or stand-alone therapeutic target in MDD.