Gut and oral microbiome modulate molecular and clinical markers of schizophrenia-related symptoms: A transdiagnostic, multilevel pilot study

•The oral microbiome is significantly altered in psychiatric disease, with an enrichment in pathogenic taxa like Veillonella and Prevotella, and can accurately classify psychiatric phenotype.•The gut microbiome of psychiatric cases exhibits significantly greater heterogeneity of microbial diversity than in healthy controls.•Gut and oral microbiota differentially associate with the severity of positive and general PANSS scores.•Psychiatric disease is characterized by a dysregulation of the relation between the microbiome and peripheral and hippocampal inflammation.•Associations between microbiota and inflammatory markers in the periphery and hippocampus identify potential gut-brain axis mechanisms.