Researchers from the University of Gothenburg and Swedish probiotic company BioGaia AB have identified a previously unknown function of gut bacteria – their ability to produce bioactive serotonin, a key signalling molecule in the microbiota–gut–brain axis.
The findings, published in the peer-reviewed journal Cell Reports, demonstrate for the first time that specific bacterial strains in the intestine can directly synthesise biologically active serotonin, rather than simply influencing its production by human gut cells.
While serotonin is widely recognised as a neurotransmitter in the brain, more than 90 percent of the body’s serotonin is produced in the gastrointestinal tract, where it plays a critical role in regulating intestinal movement via the enteric nervous system. Until now, it had been unclear whether gut bacteria were capable of producing serotonin themselves.
In the current study, researchers identified two bacterial strains – Limosilactobacillus mucosae and Ligilactobacillus ruminis – that were able to work together to produce serotonin in preclinical models. Both strains belong to the former Lactobacillus genus.
The researchers found that these bacteria increased intestinal serotonin levels, enhanced colon innervation and normalised transit time in models of serotonin deficiency.
‘It is incredibly fascinating how the bacteria in the intestine can produce bioactive signalling molecules that affect health,” said Professor Fredrik Bäckhed, Professor of Molecular Medicine at the Sahlgrenska Academy at the University of Gothenburg and a lead investigator on the study.
The study also examined stool samples from individuals with irritable bowel syndrome (IBS), a condition classified as a disorder of gut–brain interaction. Participants with IBS were found to have significantly lower levels of L. mucosae compared to healthy controls. The researchers further confirmed that this specific strain possesses the enzyme required for serotonin production.
‘Our results indicate that certain gut bacteria can produce bioactive serotonin and thus play an important role in gut health and open new avenues for the treatment of functional bowel diseases such as IBS,’ Prof Bäckhed said.
BioGaia’s Chief Scientific Officer, Gianfranco Grompone, said the findings could have broader implications beyond gastrointestinal health.
‘With this outstanding discovery, together with the University of Gothenburg, we have been able to identify that gut bacteria can produce bioactive signalling substances such as serotonin, which will not only help us to better understand how the gut microbiome signals the brain and potentially our behaviour, but also open the way for the development of innovative probiotic products in the gut health and mental health spaces,’ he said.
The discovery adds to growing global research exploring the role of the microbiome in systemic health, including its influence on neurological, metabolic and inflammatory pathways.
