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Fundamental research

18 - 07 - 2017

The intestinal microbiota: a health factor not to be neglected

The intestinal microbiota, made up of all the microorganisms present in the digestive tract, influence in many ways our health, notably our metabolism and our mood. Here is an overview of some of the animal experiments that allowed these discoveries.


The intestinal microbiota: a health factor not to be neglected

It is a genuinely "fashionable" word, and for good reason. The intestinal microbiota, commonly called microbiota, consists of all the microorganisms that colonize our digestive system. They can influence in many ways our health by interacting with our organs.

Although its existence has been known for more than a century, the intestinal microbiota has been increasingly studied thanks to the development of new research techniques that have led to a better understanding and description of the interactions between this microbiota and its host - our organism.


A microbiota of our own


The intestinal microbiota consists of about a thousand billion microorganisms, mainly bacteria, two to ten times more than the number of cells that make up our bodies. The total weight of these organisms represents about 2 kilos.

Each individual has a specific mix of microorganisms of its own, both inherited at birth and influenced by the environment (by food, hygiene, antibiotics, etc ...).



Obesity & inflammatory bowel diseases: the key role of the microbiota



The influence of the intestinal microbiota on the metabolism was very quickly demonstrated, at first, in the mouse. In fact, the transfer of the microbiota from an obese mouse to a healthy mouse is sufficient to cause an important weight gain in the latter. And conversely, when the intestinal microbiota from a healthy mouse is transplanted to an obese mouse, the animal will tend to lose weight. Recently, a study published in the journal Cell Reports, also conducted in mice, showed that the composition of the microbiota can predict how the body will respond to an unbalanced diet. Thus, in addition to genetic and environmental factors, the microbiota should also be taken into account when assessing the risk of obesity, diabetes or cardiovascular diseases.

Characterized by an inflammation of the digestive tract, inflammatory Bowel Diseases (IBD), such as Crohn's disease or ulcerative colitis, are closely linked to the intestinal microbiota. In 2016, a study by INSERM researchers showed in mice that microbiota abnormalities in IBD were both the cause and consequence of the visible inflammation, and that these diseases resulted from both genetic, immune and microbial factors.



Strong connections between the brain and the microbiota



In addition to its effects on the digestive system, the intestinal microbiota also influences on the brain. The intestinal nervous system, which alone accounts for 200 million neurons, works 80% of the time in the direction of the intestine to the brain, a bit like a "second brain". The hypothesis that a change in the microbiota can modify the information transmitted to the brain has therefore been laid down by the scientific community. The role of the microbiota is now evoked in several psychiatric diseases, including autism, schizophrenia and depression. In the case of autism, it was shown that mice could develop anxiety and self-mutilation behaviours if their microbiota was modified. However, symptoms improved after an antibiotic treatment.

Last May, a new mouse study equally showed an unexpected link between blood vessels in the brain and bacteria in the intestines, suggesting that some people may be at higher risk for stroke because of the type of bacteria composing their microbiota.

More recently, researchers at the University of Florida discovered that bone marrow is part of the connection between the brain and the intestinal microbiota. Again, using a murine model, they discovered that the brain could alter the composition of the microbiota by communicating with the immune cells of the bone marrow. This discovery, which has yet to be studied in detail, could lead to new treatments, in particular for inflammatory and metabolic diseases. This study would also influence the treatments with immunosuppressants, because of their impact on the microbiota, which could then lead to other diseases.


And tomorrow?


Thanks to these numerous studies, promising new therapies are emerging: novel antibiotics, probiotics, prebiotics, but also faecal transplants of the microbiota from a healthy individual to a sick patient. This last approach has already shown some success against intestinal infections.

It should be noted that most studies of the intestinal microbiota are carried out in mice. This is indeed an ideal experimental model for this type of work: mice are easily genetically modifiable, grow quickly and can easily be made axenic, ie raised in sterile bubbles so they lack any type of intestinal microbiota.


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