A large study of two European populations ― Belgian and Dutch people ― has discovered that several species of gut bacteria are missing in people with depression (1). The results were published on 4 February in Nature Microbiology and although they do not prove cause and effect, they do add to the mounting evidence of a so-called ‘gut–brain axis.’ The findings suggest certain substances made by gut microbes could affect nerve cell function in humans ― and therefore, mood.
The gut microbiome has become increasingly recognised as an important indicator of health and disease. Links between specific microbes and host health, genotype, and diet have been identified. Furthermore, this complex microbial ecosystem is responsible for performing numerous crucial functions within the digestive system and immune system. Disturbances in the gut microbiome have been associated with a number of physical pathologies from inflammatory bowel disease to diabetes, obesity, atherosclerosis, and even cancer. However, the connection between the gut and mental health is one of the most intriguing and often highly controversial.
Researchers led by Prof Jeroen Raes, a microbiologist from the Catholic University of Leuven in Belgium, examined the microbiome of 1054 Belgians and 1064 Dutch people. But first, to figure out why gut bacteria might be linked to mood, they developed a computational technique to identify certain gut bacteria that interact with the human nervous system. They put together a list of 56 substances and by studying the genomes of bacteria isolated from the human gastrointestinal tract, were able to catalogue the neuroactivity of more than 500 gut species.
The large study found that two species of gut bacteria — Dialister and Coprococcus — are lacking in people with depression. In addition, the presence of Faecalibacterium and Coprococcus bacteria were consistently associated with indicators of higher quality of life but depleted in depression. Moreover, these two species were missing seven patients suffering from severe clinical depression. Coprococcus, in particular, may affect pathways related to dopamine, a neurotransmitter or chemical messenger closely associated with emotions, but there is no evidence as yet to prove a definite link to depression.
Depressed people also seemed to have more of the bacteria thought to be responsible for inflammatory bowel diseases like Crohn’s ― which suggests inflammation may play a major role in mood disorders. Interestingly, both Dialister and Coprococcus produce an anti-inflammatory compound called butyrate. In addition, other microbiotas with potentially exert anti-inflammatory effects, such as Butyricicoccus, were linked to antidepressant treatment.
Deciphering the seemingly important gut-brain connection could lead to new therapies for treating depression and other mood disorders, for example, probiotics or oral bacterial supplements. Novel interventions are already being tested in Switzerland. Clinical neuroscientist Dr André Schmidt from the University of Basel has already begun a clinical trial involving a single faecal transplant to restore the gut microbiome in 40 depressed patients. The mental health and microbiota of patients will be assessed before and after the treatment.
Raes and colleagues are also preparing for the next round of the Flemish Gut Flora Project planned to start next spring. The project is aiming to analyse the microbiome of more than 5000 individuals.
(1) Valles-Colomer, M et. The neuroactive potential of the human gut microbiota in quality of life and depression. Nature Microbiology (2019). DOI: 10.1038/s41564-018-0337-x