Altered oral microbiome composition in mental disorders: a systematic review and meta-analysis

Emerging research underscores the gut-brain axis in mental disorder pathophysiology, yet the oral microbiome's contribution to mental health remains underexplored. Elucidating oral microbial signatures in mental and neurological disorders may reveal novel pathobiological mechanisms and advance biomarker discovery for precision diagnostics and microbiota-targeted interventions. This systematic review and meta-analysis investigates oral microbiota alterations across 6 different mental disorders, by synthesizing data from 20 case-control studies retrieved from PubMed, Embase, and Cochrane Library. Relative microbial abundance and beta diversity indices were extracted from between-group comparisons. Random-effects meta-analyses were conducted for alpha diversity to characterize microbiota differences between patients and controls. Key findings included a significantly higher Simpson Index in patients (SMD = 0.42; 95% CI, 0.25 to 0.60) compared to controls. Beta diversity varied significantly only in SZ and MDD. Condition-specific variations in microbial abundance were observed: Rothia enrichment in ASD, overrepresentation of H2S-producing genera in SZ, and reduced Solobacterium and Leptotrichia in MDD. Collectively, the meta-analytical synthesis suggests alterations in oral microbiota diversity across mental disorders. Disease-associated microbial shifts highlight the oral microbiome as a candidate factor warranting further investigation for potential diagnostic applications and microbial-targeted therapeutic strategies.