A new study published in Neuroscience has discovered that young adults with major depressive disorder (MDD) do not have significant changes in their overall gut microbiome compared to healthy people. However, the study also identified some specific bacteria and functions that are different between the two groups, some of which are related to the severity of depressive and anxiety symptoms.
The gut microbiome is the complex community of bacteria and other microbes that live in the digestive tract. It can influence the brain through various pathways, such as producing chemicals that affect mood and cognition, and activating the vagus nerve, which is a connection between the gut and the brain.
Previous studies have suggested that the gut microbiome may play a role in the development and treatment of MDD, but the results are inconsistent. Furthermore, only a few studies have focused on the gut microbiome of young adults with MDD, who are more vulnerable to chronic and severe depression.
Researchers from the Renmin Hospital of Wuhan University in China sought to address this gap. Led by Mian-mian Chen, the study team recruited 40 young adults with MDD and 42 healthy controls from China’s Early Warning System and Comprehensive Intervention for Depression project, and collected stool samples from them.
Chen’s team utilized a laboratory technique known as shotgun metagenomic sequencing, which allows for a comprehensive and high-resolution analysis of the gut microbiome by sampling all the genes present in the samples.
Questionnaires were administered to collect data on factors that could influence the results, known as confounding factors. These included sex, age, body mass index, diet, alcohol and cigarette consumption, and bowel movement quality. They also controlled for other potential confounding factors including exercise and defecation frequency.
The researchers compared the diversity, structure, and function of the gut microbiome between the two groups, and identified specific bacterial taxa (types) and functional modules (specialized groups).
They found that the overall gut microbiome was not significantly different between MDD and healthy controls, suggesting that depression does not alter the general composition and function of the gut microbiome in young adults.
However, Sutterellaceae bacteria decreased in the MDD group, but the “mechanism underlying this decrease in abundance in MDD is still unknown and requires further study”.
Chen and colleagues also found 15 species of bacteria that differed between the young adults with MDD and the healthy young adults, mainly belonging to the groups Clostridium, Eubacterium, and Ruminococcus.
“We found that species enriched in healthy controls mainly belonged to the Clostridium cluster XIVa, an important short-chain fatty acid-synthesizing bacteria that benefits the human body by affecting the immune system,” the authors explained.
Some of these bacterium types were correlated with symptom scale scores, such as Sutterellaceae with anxiety, Ruminococcus with depression, and Eubacterium with physical symptoms.
They also found an increase in a specialized microbe group related to degradation of an amino acid, known as cysteine, in MDD. “Increased cysteine degradation in the MDD group might further affect oxidative stress and affect the transmission of cellular signals … by activating inflammatory and oxidative pathways and impairing neurogenesis, synaptic plasticity, neuroprotection,” Chen and colleagues noted.
Some limitations are to be recognized. For example, chemical stabilization of samples meant that further analysis using a technique called metabonomics could not be conducted in order to determine metabolites and their concentrations to better understand the underlying biochemical activity. Additionally, dietary preference and education level were not well-controlled for, which may influence the composition of gut microbiome.
The study, “Young Adults with Major Depression Show Altered Microbiome”, was authored by Mian-mian Chen, Peilin Wang, Xin-hui Xie, Zhaowen Nie, Shu-xian Xu, Nan Zhang, Wei Wang, Lihua Yao and Zhongchun Liu.