Serotonin is usually thought of as a brain chemical — the neurotransmitter associated with mood, wellbeing, and the mechanism behind most antidepressants. What is less widely known is that approximately **95% of the body's serotonin is made in the gut**, not the brain. There, it acts as a critical signalling molecule, regulating the rhythmic contractions that move food through the intestines.
Now, researchers have discovered something that changes how we understand where that gut serotonin comes from — and it opens a new door for treating one of the world's most common digestive disorders.
**The Discovery**
A study published in late 2025 and reported widely in early 2026 identified two bacterial species living in the human gut that are capable of **directly synthesising serotonin** — not just indirectly stimulating the gut cells that produce it, but manufacturing the chemical themselves.
The bacteria are:
🦠 ***Limosilactobacillus mucosae*** — a Lactobacillus species found in the intestinal lining 🦠 ***Ligilactobacillus ruminis*** — a related species with a similar metabolic capability
Working together, these two bacteria can produce serotonin from a precursor molecule called **5-hydroxytryptophan (5-HTP)** — effectively running part of the serotonin synthesis pathway that was previously thought to occur only in specialised gut lining cells called **enterochromaffin (EC) cells**.
**The Mouse Studies**
The research was conducted using **germ-free mice** — animals bred in sterile conditions without any gut microbiome. These mice naturally have very low levels of intestinal serotonin, because they lack the bacterial community that, it turns out, contributes to serotonin production.
When researchers introduced the two serotonin-producing bacteria into these germ-free mice, the results were striking:
📈 Intestinal **serotonin levels increased** 🧬 The number of **nerve cells in the colon increased** ⏱️ **Gut motility normalised** — food moved through the intestines at a healthy rate
The normalisation of gut motility is particularly significant. Abnormal gut motility — food moving too fast or too slowly through the intestines — is one of the defining features of **Irritable Bowel Syndrome (IBS)**, which affects an estimated 10–15% of the global population and causes chronic abdominal pain, bloating, constipation, diarrhoea, or alternating patterns of both.
**The IBS Connection**
Perhaps the most clinically compelling finding: people with IBS have been found to have **lower levels of *Limosilactobacillus mucosae*** in their gut microbiome compared to healthy controls.
If one of the key bacteria responsible for gut serotonin production is depleted in people with IBS — and if restoring it via targeted probiotic treatment or dietary intervention can normalise serotonin levels and gut motility — then this discovery could lead to a genuinely new therapeutic approach for a condition that currently has no cure and for which treatment is largely symptomatic.
IBS affects hundreds of millions of people worldwide. Current treatments include dietary changes (particularly low-FODMAP diets), antispasmodics, laxatives, antidiarrhoeal medications, and, in some cases, antidepressants prescribed for their effects on gut serotonin signalling. These help many patients but fail many others. New mechanisms translate to new targets — and new hope.
**Why This Matters Beyond IBS**
The gut-brain axis — the two-way communication system between the intestines and the central nervous system — is one of the most active areas in neuroscience and gastroenterology. Emerging research has connected the gut microbiome to conditions ranging from depression and anxiety to Parkinson's disease.
The discovery that gut bacteria can directly produce serotonin adds a new layer to this understanding. It raises questions that researchers are now actively pursuing:
🤔 Does bacterial serotonin production in the gut influence mood and mental health, as well as bowel function? 🤔 Can targeted probiotic formulations restore serotonin-producing bacteria in people whose levels have been depleted by antibiotics, illness, or stress? 🤔 Are there other neuroactive chemicals that gut bacteria can produce directly, beyond what is currently known?
The research team noted that while these findings are currently based on preclinical (mouse) studies, the identification of the bacteria and the mechanism provides a clear translational pathway. Human trials using targeted probiotic interventions are the logical next step.
**The 95% We Ignore**
There's something slightly surreal about this discovery: the chemical most associated with happiness and mental wellbeing is, in the largest quantities, made in the digestive system. And it turns out that tiny bacteria living in the lining of our intestines have a significant hand in making it.
The implications reach from the basic science of neurochemistry to the practical treatment of common, chronic, undertreated digestive disorders that cause daily suffering for one in ten people.
The gut is stranger and more capable than we gave it credit for.
And the bacteria living in it, it turns out, are quietly contributing to the chemistry of how we feel. 🔬
*Sources: Science Daily (March 13, 2026) · New Atlas · Microbiome Post · EurekAlert! · Nutrition Insight · News-Medical.net · Aesthetic Medical Practitioner*
