Table 2.

A summary of the key experimental/animal studies included in this review.

First Author/Year	Type of Study	Findings and Comments
Zhu, Guo, 2020 [42]	Animal (mice)	FMT from schizophrenic patients into mice treated with antibiotics resulted in psychomotor hyperactivity and impaired learning and memory. Elevation of the kynurenine–kynurenic acid pathway of tryptophan degradation in both the central and peripheral nervous systems. Elevated basal extracellular dopamine in prefrontal cortex and 5-HT in hippocampus.
Zheng, 2019 [44]	Animal (mice) and Human (schizophrenic vs. controls)	Altered amino acid and lipid metabolisms, along with disruptions in the glutamate–glutamine–GABA cycle and decreased brain glutamate. Veillonellaceae OTU191 had a negative correlation with PANNS scores, whereas Bacteroidaceae OTU172, Streptococcaceae OTU834 and two Lachnospiraceae OTUs (477 and 629) had a positive correlation with PANSS scores.
Li, 2021 [49]	Human (schizophrenic vs. controls)	Relative abundance of Ruminococcus and Roseburia was significantly reduced at the genus level, while the abundance of Veillonella was significantly increased in schizophrenic patients compared to the controls.
Nguyen, 2019 [53]	Human (schizophrenic vs. controls)	Proteobacteria were relatively lower in schizophrenic subjects compared to the controls. At the genus level, Anaerococcus was relatively higher in schizophrenic subjects, whereas Haemophilus, Sutterella and Clostridium were reduced. In schizophrenic patients, the abundance of Ruminococcaceae was associated with reduced severity of negative symptoms. Bacteroides was correlated with more severe depressive symptoms.
Shen, 2018 [48]	Human (schizophrenic vs. controls)	Abundance of Proteobacteria (at the phylum level) was significantly higher in schizophrenic subjects. At the genus level, the relative abundance of Succinivibrio, Megasphaera, Collinsella, Clostridium, Klebsiella and Methanobrevibacter was significantly increased, while the abundance of Blautia, Coprococcus and Roseburia was reduced. Numerous metabolic pathways were significantly different between the healthy controls and schizophrenic subjects, such as vitamin B6 and fatty acid.
Ma, 2020 [50]	Human (FEP vs. schizophrenic vs. controls)	Both first-episode psychotic (FSCZ) patients, and chronically antipsychotic-treated schizophrenic subjects (TSCZ) had marked changes in gut microbiome composition in certain taxa, including Christensenellaceae, Enterobacteriaceae, Pasteurellaceae, Turicibacteraceae at the family level and Escherichia at the genus level. Major disturbances in the gut microbiome composition in TSCZ compared to FSCZ patients (eg. Enterococcaceae and Lactobacillaceae). Certain schizophrenia-related microbiota correlated with the right middle frontal gyrus volume, which was abnormal in schizophrenic subjects.
Pyndt Jørgensen, 2015 [63]	Animal (rats)	Hyperactivity linked to an elevation in Lachnospiraceae and Clostridiaceae. At the genus level, it was related to increased Roseburia, Clostridium and Odoribacter.
Dunphy-Doherty, 2018 [64]	Animal (rats)	Socially isolated rats had altered microbiota composition with elevated Actinobacteria and reduced Clostridia class compared to controls. Differences were also observed at the genus level. Positive correlations were seen between microbiota and hippocampal IL-6 and IL-10, conditioned freezing and open-field exploration. Adverse early life stress resulting from continuous social isolation increased ‘anxiety-like’ behaviour and impaired associative learning and memory that went along with alterations to gut microbiota, reduced hippocampal IL-6 and IL-10 and neurogenesis.
He, 2018 [67]	Human (high-risk (HR) subjects vs. ultra-high-risk (UHR) subjects vs. health controls (HC))	Increased Clostridiales, Lactobacillales and Bacteroidales were noted in the faecal samples of UHR subjects compared to the other two groups. Increased production of short-chain fatty acids (SCFAs), as indicated by changes in microbiota composition, which can lead to the activation of microglia and disruption of membrane metabolism.
Yuan, 2018 [68]	Human (FEP vs. healthy controls (HC))	FEP subjects had significantly reduced numbers of faecal Bifidobacterium spp., Escherichia coli and Lactobacillus spp. Significantly higher numbers of faecal Clostridium coccoides group in the patient group. After 24-week risperidone treatment, significant increases were noted in body weight, BMI, fasting blood glucose, triglycerides, LDL and a major elevation in the numbers of fecal Bifidobacterium spp. and E. coli. Additionally, significant decreases in the numbers of faecal Clostridium coccoides group and Lactobacillus spp.
Zhang, 2018 [69]	Human (schizophrenic (SC) vs. healthy controls (HC))	Increased abundance of harmful bacterial (Proteobacteria) and decreased short-chain fatty acid (SCFA)-producing bacteria, such as the Faecalibacterium and Lachnospiraceae genera in schizophrenic subjects. Relative reduction in alpha diversity and altered composition in the gut mycobiota. Higher levels of Chaetomium and a lower level of Trichoderma in the schizophrenic group.
Zhu, Ju, 2020 [70]	Human (medication-free psychotic vs. controls) and animals (mice)	Psychotic subjects had a higher alpha diversity and higher beta diversity. Differences in short-chain fatty acid synthesis, tryptophan metabolism and synthesis/degradation of neurotransmitters associated with schizophrenia. FMT of a schizophrenia-enriched bacterium, Streptococcus vestibularis, induced deficits in social behaviours and altered neurotransmitter levels in peripheral tissues.
Schwarz, 2018 [62]	Human (FEP vs. healthy controls)	Elevated Lactobacillus group bacteria in FEP-subjects that significantly correlated with severity along different symptom domains. FEP subjects with the highest microbiome alterations showed a poorer response after up to 12 months of treatment.