Journal of affective disordersSep 26, 2020

Gut bacteria types and tryptophan metabolism differ in bipolar disorder patients during a major depressive episode

Gut-Brain Axis Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 31, 2026

Abstract

The abundance of the phylum Bacteroidetes was significantly reduced in bipolar disorder (BPD) patients compared to healthy controls.

The abundance of Actinobacteria and Firmicutes was significantly increased in BPD patients.
A total of 38 species were found to be significantly increased, while 6 species were significantly decreased in the BPD group.
Two specific gene functions related to microbial tryptophan metabolism were significantly lower in BPD patients.
Five other gene functions were found to be significantly higher in the BPD group.
Gut microbiota classification at the genus level achieved a high accuracy, with an area under the receiver operating characteristic curve of 0.997.

Simplified

BACKGROUND: The microbiome-gut-brain axis, especially the microbial tryptophan biosynthesis and metabolism pathway (MiTBamp), is closely connected to bipolar disorder with current major depressive episode (BPD).

METHODS: We performed shotgun metagenomics sequencing (SMS) of faecal samples from 25 BPD patients and 28 healthy controls (HCs). Except for the microbiota taxa and MiTBamp analyses, we also built a classification model using the Random Forests (RF) and Boruta algorithm to find the microbial biomarkers for BPD.

RESULTS: Compared to HCs, the phylum Bacteroidetes abundance was significantly reduced, whereas that of the Actinobacteria and Firmicutes were significantly increased in BPD patients. We also identified 38 species increased and 6 species decreased significantly in the BPD group. In the MiTBamp, we identified that two Kyoto Encyclopedia of Genes and Genomes (KEGG) orthologies (KOs) (K00658 and K00837) were significantly lower in the BPD, and five KOs (K01696, K00382, K00626, K01667, and K03781) were significantly higher in the BPD group. We also identified significant genera and species which were closely related to these KOs. Finally, RF classification based on gut microbiota at the genus level can achieve an area under the receiver operating characteristic curve of 0.997.

LIMITATIONS: The features of cross-sectional design, limited sample size, the heterogeneity of bipolar disorders, and a lack of serum/plasma tryptophan concentration measurements.

CONCLUSIONS: The present findings enable a better understanding of changes in gastrointestinal microbiome and MiTBamp in BPD. Alterations of microbes may have potential as biomarkers for distinguishing the BPD patients form HCs.

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Gut bacteria types and tryptophan metabolism differ in bipolar disorder patients during a major depressive episode

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