Gut bacteria shown to produce serotonin, revealing new pathways for IBS therapy.
Researchers at the University of Gothenburg in Sweden have uncovered new details about how gut bacteria interact with the body in people with irritable bowel syndrome (IBS). Their experiments show that gut bacteria are capable of producing serotonin, a key signaling molecule in the digestive system. The discovery could open the door to new treatment options in the future.
IBS, or Irritable Bowel Syndrome, is a common gastrointestinal condition that affects more women than men and causes symptoms such as abdominal pain, constipation, or diarrhea. While its exact cause remains unclear, both the gut microbiome and serotonin appear to play major roles in its development.
Although serotonin is best known as a neurotransmitter in the brain, more than 90 percent of it is actually produced in the digestive tract. In the gut, it regulates muscle contractions and bowel movements through the enteric nervous system, often referred to as the “gut-brain.”
Earlier studies have shown that gut bacteria can influence how much serotonin the host produces, but until now, it was not known whether the bacteria themselves were capable of generating biologically active serotonin.
Bacteria that produce serotonin identified
In the current study, published in the journal Cell Reports, the researchers have identified two bacteria that together can produce serotonin: Limosilactobacillus mucosae and Ligilactobacillus ruminis.
When the bacteria were introduced into germ-free mice with serotonin deficiency, the levels of serotonin in the gut increased, as did the density of nerve cells in the colon. The bacteria also normalized the intestinal transit time.
“It is incredibly fascinating how the gut bacteria can produce bioactive signaling molecules that affect health,” says Fredrik Bäckhed, Professor of molecular medicine at Sahlgrenska Academy at the University of Gothenburg, and one of the leading authors of the study.
Potential for new IBS treatments
The researchers were also able to see that people with IBS had lower levels of one of the bacteria (L. mucosae) in their stool compared to healthy individuals, and that this bacterium also has the enzyme required for serotonin production.
Magnus Simrén is a Professor of medical gastroenterology at Sahlgrenska Academy at the University of Gothenburg:
“Our results indicate that certain intestinal bacteria can produce bioactive serotonin and thus play an important role in intestinal health and open new avenues for the treatment of functional gastrointestinal disorders such as IBS,” he says.
“Our findings indicate that intestinal bacteria can form signaling substances such as serotonin, which may be the key to understanding how the intestine and its inhabitants can affect our brain and the behavior,” concludes Fredrik Bäckhed.
Reference: “Identification of human gut bacteria that produce bioactive serotonin and promote colonic innervation” by Chiara H. Moretti, Estelle Grasset, Jiaying Zhu, Gaohua Yang, Louise E. Olofsson, Muhammad Tanweer Khan, Per-Olof Bergh, Jee-Hwan Oh, Annika Lundqvist, Tom van Gils, Manuela Krämer, Lisa M. Olsson, Piyush Patel, Matthias Mitteregger, Daysi Espinola Monges, Chinmay Dwibedi, Kimberly A. Krautkramer, Nienke Koopman, Marcus Henricsson, Andrew J. Macpherson, Thue Schwartz, Gianfranco Grompone, Jan-Peter van Pijkeren, Valentina Tremaroli, Stefan Roos, Magnus Simrén and Fredrik Bäckhed, 20 October 2025, Cell Reports.
DOI: 10.1016/j.celrep.2025.116434
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