The intestinal microbiota in figures
1,5 In kilos, the weight of all the microorganisms that inhabit our intestine
10 to 1000 The number of bacteria present per milliliter in the stomach
10,000 to 10 million The number of bacteria present per milliliter in the small intestine
10 billion to 10,000 billion The number of bacteria present per milliliter in the colon
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By repeating it, the phrase has practically become a commercial slogan. The intestine and the billions of bacteria that inhabit it, in other words our intestinal microbiota (read framed), are a form of the second brain. As with metabolism and in particular obesity, the supposed influence of this organ on the brain comes from studies which compared the behavior of mice born without microbiota, called axenic, with that of animals born normally. “By observing the movements of the animals in their cages, we noticed that the axenic mice were less adventurous than their congeners: they stayed in the nooks and did not go on an adventure to explore the environment in which they found themselves”, explains Professor Jacques Schrenzel from the Bacteriology Laboratory of the Geneva University Hospitals. This tendency to withdraw into oneself and seek protection has been equated with a greater propensity for anxiety and fear. Mice born with a microbiota, because they were more reckless, were considered mentally more mature.
“Reset” the intestinal flora
To explain these differences in behavior, scientists point to a “dysbiosis” of the intestinal microbiota. In some individuals, there is an imbalance of the latter with dominant microbial species that proliferate in an anarchic manner in the intestine. And these imbalances would impact the functioning of the brain. “The exact mechanisms by which this dysbiosis has an effect on the brain are not yet clearly established,” admits Jacques Schrenzel. There are several potential direct and indirect pathways through which the gut microbiota can modulate the gut-brain axis, such as the endocrine (cortisol), immune (cytokines) and neuronal (vagal and enteric nervous system) pathways. But nothing is fully demonstrated today. ”
Even if the mechanisms in play are not yet fully explained, the hypotheses to correct these imbalances are well underway: the introduction of the microflora of a healthy individual should help to recolonize the system with a microbial model more in line with the well-being, either by transplanting a new microbiota, or by allowing the host to “reset” its own microflora to a state prior to the disease. How to get there? Quite simply by imitating the model observed in animals. When a mouse artificially devoid of microbiota eats the feces of its healthy counterparts, its behavior and metabolism change and become similar to the normal mouse. What is proposed to rebalance the human microbiota is identical: it involves making someone who is sick “ingest” the feces of a healthy person (coprophagia). Scientific code name: faecal transplantation, a process that (fortunately) bypasses the mouth. It would thus be possible to rebalance the system.
From depression to schizophrenia
With this therapeutic approach, the hope is to be able to treat many psychiatric pathologies. In a study published in 2019 in Nature Microbiology, researchers from the Catholic University of Louvain in Belgium have thus identified two bacteria, Coprococcus and Dialister, which were systematically found in the microbiota of people suffering from depression, regardless of antidepressant treatment.[1] Reducing their intestinal influence would thus be a new avenue to fight against the disease. Another study, published in the Journal of the American Medical Association, establishes a link between having been hospitalized for an infection during childhood and an increased risk of developing a mental disorder thereafter.[2] According to the authors: the inflammatory nature of the infection, but also the potentially disruptive role of antibiotics on the microbiota. “Studies like these exist for many pathologies falling within the scope of psychiatry,” comments Professor Schrenzel. For schizophrenia and autism in particular, but also for degenerative diseases such as Parkinson’s disease. However, we must remain cautious because this research certainly shows associations, but in no case demonstrated causalities. The interactions are very numerous and it remains difficult to isolate the mechanisms at work. ”
Towards psychobiotics?
These limitations do not prevent some researchers, Irish in particular, from predicting the advent of “psychobiotics” to treat these pathologies. “It is a question of making a sick individual eat microorganisms that they will not be able to digest and which could restore a healthy functioning of the microbiota”, explains Jacques Schrenzel. If the approach is attractive on paper, the Geneva professor nevertheless thinks that we are still far from having sufficient proof of the effectiveness of the process: “For the moment, these are still very preliminary hypotheses which have a very low level of evidence. ” In other words, research and its clinical applications are still in their infancy. “We must be right,” concludes the expert. Those who say the microbiota will fix everything are going too far. Those who think he has no influence are too pessimistic. The future and patience will tell us in which areas it has an influence. But that it has a considerable influence on health, including mental health, is not in doubt. “
The intestinal microbiota at a glance
The microbiota is the set of micro-organisms – bacteria, viruses, parasites, non-pathogenic fungi, called commensals – which live within an organism without harming it. In humans, there are several, in the skin, mouth, vagina, etc. The gut microbiota is the most important of these, with 1012 to 1014 micro-organisms: as many as the number of cells that make up our body!-
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Published in Esprit (S), the review of Pro Mente Sana, November 2020.
[1] Valles-Colomer M, Falony G, Darzi Y, et al. The neuroactive potential of the human gut microbiota in quality of life and depression. Nat Microbiol 4,623-32 (2019). Disponible sur : https://www.nature.com/articles/s41564-018-0337-x.
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