Recurrent abdominal pain, diarrhea or constipation without visible lesion: the
irritable bowel syndrome (IBS) affects a large part of the population, especially women, and remains difficult to provide lasting relief. Behind these symptoms, doctors have long suspected a mix of disrupted motor skills, abnormal nerve signals and alterations in the intestinal microbiota.
A team from the University of Gothenburg has just taken an unexpected step: it has identified intestinal bacteria able to make their own
serotonina key chemical messenger of transit. Published in the journal Cell Reportstheir work suggests that by precisely targeting these microbes, we could one day correct certain functional digestive disorders such as IBS.
Microbiota, serotonin and irritable intestine: an increasingly precise link
In our body, more than 90% of the
serotonin is produced in the intestine, not the brain. This molecule controls intestinal movements via the enteric nervous system, sometimes nicknamed “intestinal brain” or “second brain”, and participates in the perception of digestive pain. Previous research had shown that the microbiota modulated the amount of serotonin produced by the host, without knowing whether the microbes produced it directly.
The researchers used mice raised without microbes and lacking peripheral serotonin. After introducing the bacteria studied, the level of serotonin in the intestine increased, as did the density of neurons in the colon, and transit time returned to normal. “It’s incredibly fascinating to see how gut bacteria can produce bioactive signaling molecules that influence health“, comments Fredrik Bäckhed, professor of molecular medicine, in a press release from the University of Gothenburg.
Two bacteria identified as serotonin producers
At the heart of this progress, two species of human lactobacilli: Limosilactobacillus mucosae And
Ligilactobacillus ruminis. Together, they are capable of transforming a precursor, 5-hydroxytryptophan (5-HTP), into bioactive serotonin. In mice lacking serotonin, their colonization increased the serotonin measured in the stools and in the wall of the colon, while strengthening the nerve network that controls intestinal movements.
The scientists then compared 147 people with IBS to 27 healthy volunteers. Serotonin levels in blood and stool did not differ markedly, but the abundance of Limosilactobacillus mucosae was significantly lower in patients, and this bacteria was associated with fewer hard stools. “Our results indicate that certain gut bacteria can produce bioactive serotonin and therefore play an important role for gut health, while opening new avenues for the treatment of functional digestive disorders like IBS.“, explains Magnus Simrén, professor of medical gastroenterology.
Towards treatments targeting the bacteria of irritable bowel syndrome
The results suggest that these microbes mainly supply local serotonin in the intestinal lumen, more than circulating serotonin, which could be sufficient to modify transit and innervation without directly affecting the brain. “Our work indicates that gut bacteria can form signaling substances like serotonin, which could be key to understanding how the gut and its inhabitants influence our brains and behavior.“, continues Fredrik Bäckhed.