Mental and gut health, closely linked by the microbiome

The gut communicates with different organs of the human body, including the brain. In this communication, the bacteria housed in the intestine play an important role and can interfere in our mental health. This is, among others, one of the topics to be addressed at the ninth edition of The Barcelona Debates on the Human Microbiome 2023, one of the most important scientific conferences worldwide in the field of the microbiome, which will take place on June 8 and 9 at the CosmoCaixa in Barcelona. The debate, organized by the IrsiCaixa AIDS Research Institute –a centre jointly promoted by the "la Caixa" Foundation and the Department of Health of the Generalitat de Catalunya–, has the support of "la Caixa" Foundation.

The congress, carried out in the context of the MISTRAL European project led by IrsiCaixa, is coordinated by the director of IrsiCaixa Bonaventura Clotet, the emeritus physician of the Digestive System Research Unit of the Vall de Hebron Hospital Research Institute Francisco Guarner and the principal investigator of IrsiCaixa and head of the Infectious Diseases Service of the Germans Trias i Pujol Hospital Roger Paredes. "The symposium allows us to share the latest data on the microbiome and position Barcelona at the centre of research in this scientific area", highlights Guarner.

What your gut bacteria say about your physical and mental health

For years, the scientific community has been studying how the microorganisms that make up the gut microbiota can influence the functioning of other organs. Regarding the brain, in 2019 the first study in humans describing the role of the microbiome in mental health at a population level was published. Postdoctoral researcher at the University of Trento and first author of the study Mireia Vallès-Colomer will present the results at the congress. "We saw that the bacteria that were related to a good quality of life, that is, a good condition of physical and mental health, were characterized by being anti-inflammatory and, therefore, contributed to a healthier gut environment", she explains. Specifically, what Vallès-Colomer's team observed is a predominance, at the level of the intestinal microbiome, of Coprococcus and Dialister bacteria in people with a better quality of life. In contrast, the presence of Enterotype 2 Bacteroides was associated with depression and poorer mental health.

On the same line, IrsiCaixa postdoctoral researcher Alessandra Borgognone presented at this year's Conference on Retroviruses and Opportunistic Infections (CROI) a study in which similar microbiome patterns were detected among individuals with greater neurocognitive impairment. "We observed that the microbiome profiles of individuals with autism spectrum disorder described in previous studies were similar to those of individuals in our study with neurocognitive impairment. In particular, in these individuals there was an increased presence of Sutterella and Desulfovibrio type bacteria, previously associated with cognitive impairment", details Borgognone.

Millions of compounds produced by bacteria can affect brain functionality

There are different pathways through which these patterns of microorganisms can influence mental health. In fact, the bacteria we find in the gut not only influence brain activity through their anti-inflammatory role, but also have the ability to produce and degrade compounds, such as neurotransmitters, that alter brain functioning. "When we published the paper in 2019, we dated up to 500 bacterial genomes capable of producing these neuroactive compounds, that is, which have an effect at the neuronal level. Currently, we have already described millions", explains Vallès-Colomer.

What determines the composition of our microbiome

The factors that determine which microorganisms lodge in each person's gut and, consequently, influence their physical and mental health, are very diverse. Beyond genetics, one of these factors would be the people with whom we share space and time. "In January we published a study in which we showed that individuals who live together in the same space and/or establish close physical contact share more than 10% of the gut microbiome and more than 30% of the oral microbiome. People who are strangers to each other and who have not shared any space, on the other hand, share 0%", explains Vallès-Colomer.

"The gut microbiome, although apparently it may seem to be isolated, has a close relationship with all the organs of the body and, therefore, everything that happens to us in the body has a direct effect on the composition of the bacteria in our gut. Specifically, we are studying the relationship between HIV infection, Alzheimer's disease and the post-COVID condition with the microbiome", explains Paredes. However, communication between organs and the microbiome is bidirectional. As Clotet specifies, "just as the changes that occur in our organs can influence the composition of our microbiome, the pattern of microorganisms housed in our gut can also have a direct effect on the rest of the body's organs".