Gut Bacteria and Serotonin: Hope for IBS Sufferers

A Discovery Published in Cell Reports

A team of researchers from the University of Gothenburg, in collaboration with the Swedish company BioGaia, has identified two bacteria present in the human microbiome capable of producing biologically active serotonin: Limosilactobacillus mucosae and Ligilactobacillus ruminis. The results, published in the journal Cell Reports, could transform the management of irritable bowel syndrome (IBS), a condition that affects between 10 and 15% of the world's population.

Serotonin: More Than Just a 'Happiness Hormone'

While serotonin is often associated with mood regulation, it also plays a fundamental role in digestive function. More than 90% of the human body's serotonin is produced in the gut, where it regulates muscle contractions and intestinal motility via the enteric nervous system – sometimes dubbed the 'second brain'.

Until now, it was known that certain specialized intestinal cells (enterochromaffin cells) synthesized this molecule. The novelty of this study lies in demonstrating that bacteria themselves can accomplish this synthesis, acting in concert. L. mucosae provides the enzyme necessary for the decarboxylation of 5-hydroxytryptophan, a precursor to serotonin, while L. ruminis contributes to the process.

L. mucosae Deficient in Patients with IBS

One of the most striking aspects of the study concerns patients suffering from irritable bowel syndrome. Analyses of their stool reveal significantly lower levels of L. mucosae compared to healthy individuals – and it is precisely this bacterium that possesses the key enzyme for serotonin synthesis.

Professor Magnus Simrén, co-author of the study and specialist in functional gastrointestinal disorders at the Sahlgrenska Academy, emphasizes that "these results indicate that certain gut bacteria can produce bioactive serotonin and thus play an important role in intestinal health."

Convincing Results in Murine Models

To validate their hypotheses, the researchers conducted experiments on axenic mice (devoid of any microbiome) with serotonin deficiency. Administration of the two bacteria resulted in:

• A measurable increase in intestinal serotonin levels;
• An increased density of nerve cells in the colon;
• A normalization of intestinal transit time, thereby correcting motility dysfunctions.

"It is fascinating to see that gut bacteria can produce bioactive signaling molecules that influence health," said Professor Fredrik Bäckhed, director of the study and specialist in molecular medicine at the University of Gothenburg.

A Promising Therapeutic Avenue

IBS, which manifests as abdominal pain, bloating, episodes of diarrhea and constipation, affects more women than men and remains difficult to treat. Current approaches mainly target the symptoms, without addressing the underlying mechanisms.

This discovery paves the way for microbiome-based treatments – targeted probiotics or bacterial therapies – that could restore intestinal serotonin levels in deficient patients. Clinical trials will be necessary before any therapeutic application in humans.

For the millions of patients in France, Belgium, and Switzerland who live with this chronic functional disorder, this work represents concrete and scientifically grounded hope.