Table 1.

Gut microbiome and schizophrenia/first episode of psychosis: changes in microbiome composition and possible pathophysiological mechanisms.

	Microbiome Changes	Pathophysiological Mechanisms
Schizophrenia	No changes in alpha diversity (most studies); Changes in beta diversity (consistent finding); Reduced Ruminococcus and Roseburia; increased Veillonella; Decreased Proteobacteria; Increased Proteobacteria and Fusobacteria; Firmicutes decreased; Alterations in Aerococcaceae, Bifidobacteriaceae, Brucellaceae, Pasteurellaceae and Rikenellaceae; Reduced levels of the phylum Proteobacteria and increased Actinobacteria and Bacteroidetes; Mediating factors: lifestyle changes (e.g., unhealthy diet and smoking), antipsychotic medication and age.	Increased intestinal permeability; Systemic low-level inflammation/immune system involvement/proinflammatory cytokines; Modulation of tryptophan–kynurenine metabolism; Altered amino acid and lipid metabolisms; Disruptions in the glutamate–glutamine–GABA cycle and reduced brain glutamate; Metabolic pathways: vitamin B6, fatty acid, starch and sucrose, cysteine, methionine, linoleic acid metabolism, degradation of xenobiotics; Alterations in brain structure.
First episode of psychosis (FEP)	Relative reduction in alpha diversity; Elevated Clostridiales, Prevotella and Lactobacillus ruminis; Lower numbers of faecal Bifidobacterium spp., Escherichia coli and Lactobacillus spp., and higher numbers of faecal Clostridium coccoides; Increased abundance of the harmful phylum Proteobacteria; Marked changes in Christensenellaceae, Enterobacteriaceae, Pasteurellaceae, Turicibacteraceae at the family level and Escherichia at the genus level.	Lower serum tryptophan levels; Higher kynurenic acid (KYNA) levels; Antipsychotic medication induced metabolic changes; Link between gut microbiome and changes in brain structure (grey matter volumes).