Gut Microbiome Dysregulation Across Schizophrenia Spectrum Disorders: Bacteria-, Fungi- and Virome-Level Alterations with Molecular and Immunological Implications

Schizophrenia spectrum disorders (SSD) are severe psychiatric conditions characterized by disturbances in cognition, emotion, and behavior, with increasing evidence suggesting an involvement of the gut microbiome in their pathophysiology. This PRISMA-informed structured review synthesizes 114 studies using a taxa-centered framework that maps microbial changes across SSD stages and phenotypes and serves as a structural basis for identifying cross-study patterns. Across heterogeneous cohorts, convergent alterations include depletion of short-chain fatty acid (SCFA)-producing taxa (including Faecalibacterium, Roseburia, and Coprococcus) and enrichment of potentially pro-inflammatory and fermentative taxa (such as Proteobacteria, Enterobacteriaceae, Streptococcus, Collinsella, and Desulfovibrio). These taxonomic patterns suggest potential functional alterations, including reduced SCFA availability. Reduced abundance of butyrate-producing taxa has been associated with impaired intestinal barrier function and increased microbial translocation (e.g., lipopolysaccharide), which may contribute to the activation of immune pathways, including Toll-like receptor 4 signaling and elevated inflammatory markers such as IL-6 and TNF-α. Additional alterations reported across studies include changes in lactate metabolism, bile acid profiles, aromatic amino acid metabolism, and the tryptophan-kynurenine pathway. These pathways may interact with neurobiological processes relevant to SSD, including glutamate-GABA balance, NMDA receptor function, microglial activation, and synaptic regulation, although much of the current evidence remains associative. Multi-kingdom studies and fecal microbiota transplantation models provide further support for the functional relevance of these observations, though causal relationships remain to be fully established. Overall, SSD-associated dysbiosis appears to reflect ecosystem-level metabolic alterations rather than isolated taxonomic abnormalities, supporting a Microbiota–Gut–Immune–Glia conceptual framework and highlighting the gut ecosystem as a potential therapeutic target.