A new study indicates that microbial diversity in the gut is closely related to the severity of the major depressive disorder. Compared to healthy individuals, the abundance of Bacteroides species was significantly increased in participants with moderate and severe depression, while Ruminococcus and Eubacterium were depleted mainly in participants with severe depression. The study was published in Translational Psychiatry.
Depression, also known as the major depressive disorder, is the most common form of mental illness. It is believed to be affecting more than 350 million people worldwide. The main symptoms of depression include persistent feelings of sadness, loss of interest or pleasure in activities, changes in appetite or weight, sleep disturbances, fatigue, feelings of worthlessness or guilt, difficulty concentrating or making decisions, and recurrent thoughts of death or suicide. As such, it creates an enormous social cost both on the individual suffering from it and on the society.
Novel studies have resulted in the discovery of the microbiota-gut-brain axis, a complex network that allows communication between the diverse community of microorganisms residing in the gastrointestinal tract and the brain. Since this communication is bi-directional, researchers have been examining the links between the gut microbiota composition and psychological properties, such as cognition and well-being, but also mental health.
Study author Xi Hu and his colleagues wanted to explore the differences in gut microbiota composition between persons suffering from depression, but with different levels of symptoms. They noted that previous studies have used methods for identifying bacterial species with limited accuracy and decided to use metagenome sequencing for this purpose.
Metagenome sequencing is a genomic technique used to analyze the genetic material, DNA or RNA, present in a complex mixture of microorganisms, such as those found in environmental samples or within a human body. Instead of focusing on the genetic material of a single organism, metagenome sequencing allows for the simultaneous analysis of the collective genetic material of an entire microbial community. As the goal of the procedure, in this case, was to assess microbial composition of participants’ guts, it was used on stool samples.
Study participants were 138 persons with depression of different severity – from mild to severe and 155 healthy individuals serving as controls. The severity of depression was assessed using the HAMD-17 scale. Participants who were pregnant or breastfeeding, who used antibiotics within one month before sampling, had history of alcohol or substance abuse or chronic somatic diseases or other severe psychiatric disorders were excluded from the study.
Stool samples were collected in the clinical center where the study was carried out. This was done in the morning between 7 am and 10 am. Samples were stored in sterile tubes at 4 °C, then transferred to a −80 °C refrigerator. Processing was done within 6 hours.
Results showed that, overall, at the family level, Bacteroidaceae, Lachnospiraceae, Ruminococcaceae and Prevotellaceae were the major high abundance bacterial taxa in all both healthy participants and those with depression. At the genus level, Bacteroides, Faecalibacterium, Blautia, Prevotellaceae were the major high abundance bacterial taxa in both groups.
Compared to the healthy control group, bacteria from the Bacteroides group were much more abundant in participants with moderate and severe depression. Faecalibacterium and Escherichia were decreased in the moderate depression symptom group, while Ruminococcus and Eubacterium were decreased only in the severe depression symptoms group.
When enterotypes, distinct types of bacterial communities within the human gut microbiota, were examined, results showed that an enterotype dominated by Faecalibacterium was more abundant in the healthy group, while two enterotypes dominated by Bacteroides were more abundant in the group of patients with depression.
“In this study, we found that the gut microbiota of moderate and severe major depressive disorder patients were characterized by the enrichment of Bacteroidetes, while Ruminococcus and Eubacterium were depleted in the severe patients. Consistently, the major enterotype of healthy controls was Faecalibacterium. In addition, we also identified a microbial marker panel which is capable of distinguishing major depressive disorder patients with different severity,” the researchers concluded.
The study makes an important contribution to the scientific understanding of the links between gut microbiota composition and mental health. It should, however, be noted that the study design does not allow any cause-and-effect conclusions to be made. Additionally, all samples were collected from the same regional clinical center. Results on persons from other regions or countries might not be the same.
The study, “Changes of gut microbiota reflect the severity of major depressive disorder: a cross sectional study”, was authored by Xi Hu, Yifan Li, Jing Wu, Hanping Zhang, Yu Huang, Xunmin Tan, Lu Wen, Xingyu Zhou, Peijun Xie, Oluwatayo Israel Olasunkanmi, Jingjing Zhou, Zuoli Sun, Min Liu, Guofu Zhang, Jian Yang, Peng Zheng, and Peng Xie.