# Microbial dysbiosis as a diagnostic marker in psychiatric disorders: a systematic review of gut–brain axis disruptions

## TL;DR

This review finds consistent gut microbiome changes in psychiatric disorders, suggesting potential microbial biomarkers for diagnosis.

## Contribution

The study identifies disorder-specific microbial signatures in psychiatric conditions, offering new insights for non-invasive diagnostic tools.

## Key findings

- Psychiatric disorders show reproducible gut microbial dysbiosis patterns.
- Microbiome profiling could serve as a non-invasive diagnostic adjunct in psychiatry.
- Specific bacterial phyla and genera are consistently altered in conditions like ASD, mood disorders, and schizophrenia.

## Abstract

Mental health disorders represent a major global health burden. Recent interest has surged in the microbiome–gut–brain axis, which may influence psychiatric pathophysiology. This systematic review evaluates alterations in intestinal microbiome (IM) composition between individuals with psychiatric disorders—such as schizophrenia, autism spectrum disorder (ASD), mood and eating disorders—and healthy controls, with a focus on diagnostic relevance.

We conducted a systematic review across PubMed, Scopus, CENTRAL, and PsycINFO, following PRISMA 2020 guidelines. Studies were included if psychiatric diagnoses were made using DSM-V and intestinal dysbiosis was characterised at the phylum, family, and genus levels. Only observational and interventional studies were considered. Microbial alterations were extracted and analysed both qualitatively and quantitatively. Risk of bias was assessed using NIH Quality Assessment Tools.

A total of 80 studies involving 2,691 participants met the inclusion criteria. Across disorders, consistent disruptions were observed in Firmicutes, Bacteroidetes, and Actinobacteria phyla presented as the percentage of affected patients within each disorder. Autism spectrum disorder (ASD) was associated with decreased Firmicutes (↓ 4.79%) and Bacteroidetes (↓ 3.29%) and increased Bifidobacteriaceae (↑ 5.86%) and Eggerthellaceae (↑ 5.50%). Mood disorders, including major depressive disorder and bipolar disorder, showed increased Christensenellaceae (↑ 18.1%) and decreased Ruminococcaceae (↓ 2.0%). Schizophrenia was marked by elevations in Lachnospiraceae, Christensenellaceae, and Enterobacteriaceae (↑ 11–28%) and reductions in Akkermansia and Turicibacteraceae (↓ 9–28%). Anorexia nervosa and binge eating disorder displayed profound dysbiosis, including ↓ Lactobacillus (48.5%) and complete loss of Akkermansia (100%). ADHD showed a Firmicutes/Bacteroidetes imbalance (↑ 49.8%, ↓ 56.6%). These alterations suggest microbial signatures that are both disorder-specific and partially overlapping.

Our findings highlight reproducible patterns of gut microbial dysbiosis that may represent candidate microbial biomarkers and inform future diagnostic research. Microbiome profiling has potential as a non-invasive adjunct to psychiatric diagnosis, warranting further exploration. Future longitudinal and mechanistic studies using standardised methods are essential to validate these microbial signatures and their diagnostic utility.

https://www.crd.york.ac.uk/PROSPERO/view/CRD42021254293, CRD42021254293.