About **95% of the body's serotonin** — the chemical most people associate with mood — is actually made in the gut, not the brain. The gut uses serotonin to regulate everything from bowel movements to the speed of digestion to the communication between the intestinal wall and the nervous system.
For years, scientists have known that gut bacteria can *influence* how much serotonin the gut produces. But whether bacteria could actually *make* biologically active serotonin themselves — whether microbes living in the intestines could be directly synthesising the chemical — has been much less clear.
A new study has answered that question. And it opens a door that could change how we treat some of the most common and least-understood digestive conditions in the world.
**Two Bacteria, One Discovery**
Researchers at the **University of Gothenburg, Sweden**, published findings in *Cell Reports* identifying two specific bacterial species capable of producing serotonin in the gut:
- ***Limosilactobacillus mucosae*** — a lactic acid bacterium found in the intestinal mucus lining - ***Ligilactobacillus ruminis*** — another gut-resident bacterium from the same family
Both bacteria work by **decarboxylating 5-HTP** (5-hydroxytryptophan) — a precursor molecule — converting it into fully active serotonin. This is the same basic step the human body uses to make serotonin in nerve cells, but it's been confirmed here in two strains of gut-resident bacteria.
The key question was: does the serotonin they produce actually *do anything* in the body?
**What Happened in the Mouse Studies**
To test this, the researchers used **germ-free mice** — animals raised in completely sterile conditions with no gut bacteria at all, and consequently very low serotonin levels in their intestinal tract.
When *L. mucosae* and *L. ruminis* were introduced into these mice:
- ⬆️ **Serotonin levels in the gut rose significantly** — the bacteria were genuinely producing and releasing serotonin into the intestinal environment - 🧠 **Nerve cell density in the colon increased** — the enteric nervous system (the gut's own neural network) responded to the serotonin signal and grew new nerve cells - ⏱️ **Intestinal transit time normalised** — bowel movements that had been abnormally slow in the serotonin-deficient mice returned to normal speed and pattern
The combination of these three effects — restored serotonin, more gut nerve cells, normalised bowel function — is striking. It suggests the bacteria aren't just producing a molecule in isolation, but triggering a cascade of physiological responses in the gut.
**The IBS Connection**
This is where the research becomes particularly relevant. **Irritable Bowel Syndrome (IBS)** affects an estimated **10–15% of people worldwide** — characterised by chronic abdominal pain, bloating, and disordered bowel function. It's one of the most common reasons people visit gastroenterologists, and it has no definitive cure.
One of the consistent findings in IBS research is that **people with IBS have lower levels of *L. mucosae*** in their gut microbiome compared to healthy individuals — one of the bacteria the Gothenburg team has now shown to produce serotonin.
If reduced *L. mucosae* means reduced serotonin production in the gut, and if that's contributing to the disordered bowel function that defines IBS — then restoring those bacteria could potentially restore normal gut function.
That's still a hypothesis. The research was conducted in mice, not humans, and translating results from rodent models to human treatment takes years of additional work. The researchers are careful about this.
But the mechanism is coherent. The results are compelling. And for a condition that has frustrated doctors and patients for decades, a clear biological target — the right bacteria, producing the right chemical — represents exactly the kind of lead that transforms understanding.
**The Gut-Brain Connection**
The finding adds another chapter to one of the most fascinating areas in modern medicine: the **gut-brain axis**. The gut contains over **500 million neurons** — more than the spinal cord — and communicates constantly with the brain through the vagus nerve and through chemical signals including, centrally, serotonin.
What we eat, which bacteria live in our guts, and how much serotonin those bacteria produce may shape not just digestion, but mood, cognition, and anxiety. The boundary between gut health and mental health is increasingly blurry.
Two bacteria, making the molecule that helps regulate bowel movement, nerve growth, and the conversation between the intestine and the brain. 🦠🧬💛
*Sources: ScienceDaily (March 13, 2026) · EurekaAlert · Cell Reports (University of Gothenburg) · New Atlas · Nutrition Insight*
