Several groups of gut bacteria can produce serotonin or stimulate its production, including species within Lactobacillus, Streptococcus, Escherichia coli, Klebsiella, and Lactococcus. About 90% of your body’s total serotonin is made in the gastrointestinal tract, and the bacteria living there play a surprisingly active role in that process.
Bacteria That Directly Produce Serotonin
The gut bacteria capable of making serotonin do so by expressing enzymes that convert tryptophan, an amino acid from your diet, into serotonin. Species within Lactococcus, Lactobacillus, Streptococcus, Escherichia coli, and Klebsiella have all been identified as serotonin producers. These bacteria essentially hijack the same raw ingredient your own cells use and run their own conversion process in the gut lumen.
A 2025 study published in Cell Reports identified two species with particular precision: Limosilactobacillus mucosae and Ligilactobacillus ruminis. Working together as a consortium, these bacteria synthesized serotonin in lab conditions by converting a tryptophan intermediate called 5-hydroxytryptophan. When introduced into germ-free mice that couldn’t produce their own serotonin, the bacteria raised fecal serotonin levels, increased the density of neurons in the colon, and boosted the number of serotonin-responsive neurons. This was one of the first demonstrations that specific bacterial strains could restore serotonin signaling in a living gut.
How Gut Bacteria Make Serotonin
Your own intestinal cells produce serotonin through a two-step process: first converting tryptophan into 5-hydroxytryptophan, then converting that into serotonin. Bacteria appear to use a similar but not identical pathway. No bacterial genes for the first enzyme in the human pathway (tryptophan hydroxylase) have been identified yet. Instead, bacteria seem to rely on alternative enzymes to accomplish the same chemical steps, including one that typically processes a different amino acid, phenylalanine, but can also act on tryptophan.
The key takeaway is that bacterial serotonin production depends heavily on available tryptophan. Since tryptophan is an essential amino acid that humans can only get from food, your diet directly shapes how much raw material these bacteria have to work with.
Bacteria That Influence Serotonin Without Making It
Not every bacterium involved in gut serotonin actually produces the molecule. Some species influence serotonin levels by interacting with the specialized cells in your gut lining called enterochromaffin cells, which are responsible for the bulk of intestinal serotonin production.
Bifidobacterium dentium, for example, produces acetate (a short-chain fatty acid) that can regulate components of the serotonin system across multiple tissues. In mouse studies, B. dentium’s metabolites influenced enterochromaffin cell serotonin output and were associated with measurable changes in adult behavior. Akkermansia muciniphila has also been studied for its effects on serotonin-related gene expression in both the colon and hippocampus of mice.
Then there’s Turicibacter sanguinis, which takes a different approach entirely. Rather than producing serotonin, it imports it. T. sanguinis has a transporter on its surface that closely resembles the human serotonin transporter. It actively pulls serotonin out of its surroundings, and this uptake can be blocked by fluoxetine (Prozac), the same drug that blocks serotonin reuptake in the human brain. This import ability appears to help the bacterium colonize the gut more effectively, essentially using the host’s serotonin signaling as a survival tool.
Probiotics and Serotonin Regulation
Several probiotic strains have shown the ability to modulate serotonin metabolism in research settings. Lacticaseibacillus rhamnosus and Limosilactobacillus reuteri are linked to higher concentrations of serotonin transporters in the intestine, which affects how serotonin is cleared and recycled after release. Lactobacillus acidophilus and Bifidobacterium longum have been studied for their effects on serotonin transporter expression at the genetic level.
Research on L. rhamnosus specifically found both antidepressant effects and notable influence on serotonin metabolism. These findings don’t mean popping a probiotic will reliably boost your serotonin, but they do show that the composition of your gut microbiome has measurable effects on serotonin pathways.
Tryptophan in Your Diet Fuels the Process
Since tryptophan is the sole precursor of serotonin, what you eat matters. When dietary tryptophan is restricted, Lactobacillus populations in the gut decline. Restoring tryptophan through diet selectively expands Lactobacillus colonies, which then ramp up their tryptophan metabolism. This creates a feedback loop: more dietary tryptophan feeds the bacteria that use tryptophan, and those bacteria produce more serotonin and related compounds.
Foods rich in tryptophan include turkey, chicken, eggs, cheese, nuts, seeds, tofu, and fish. The relationship between eating these foods and gut serotonin production isn’t as simple as “eat more turkey, get more serotonin,” because your body’s own enzymes compete with gut bacteria for the same tryptophan supply. Inflammation, for instance, activates an enzyme that diverts tryptophan away from serotonin production entirely, which can simultaneously reduce the tryptophan available to your gut microbes.
Does Gut Serotonin Affect Your Brain?
This is the question most people really want answered, and the relationship is more complex than popular science often suggests. Gut serotonin was long thought to stay entirely outside the brain because it can’t easily cross the blood-brain barrier. More recent evidence complicates that picture.
Serotonin produced by enterochromaffin cells gets picked up by platelets in the bloodstream and distributed throughout the body. These platelets carry serotonin to the blood-brain barrier, where it appears to regulate the barrier’s permeability. Serotonin transporters exist on the blood-brain barrier itself, suggesting that gut-derived serotonin carried by platelets can enter the fluid surrounding brain cells and influence neurotransmission through diffusion.
This doesn’t mean gut serotonin controls your mood the way brain serotonin does. The gut’s serotonin primarily regulates digestion, gut motility, nausea, and blood flow to the intestines. But the emerging picture is that gut serotonin acts as a body-wide signaling molecule affecting membrane permeability in organs and tissues, including the brain, rather than being neatly confined to the digestive system.
Serotonin Imbalances and Gut Conditions
Disruptions in gut serotonin signaling are closely tied to irritable bowel syndrome. In people with constipation-predominant IBS, mucosal serotonin concentrations are significantly higher than in healthy controls, likely because serotonin is being produced normally but not released properly into surrounding tissue. At the same time, the serotonin transporter that clears serotonin after it’s done its job shows reduced activity in constipation-predominant IBS, meaning serotonin lingers longer than it should in some areas while being trapped in others.
People with diarrhea-predominant IBS show different patterns, with changes in the enterochromaffin cells that produce serotonin. Post-infectious IBS, which develops after a bout of gastroenteritis, also involves altered enterochromaffin cell numbers. Functional constipation without IBS has been linked to reduced enterochromaffin cells overall. The picture is messy and still being sorted out, but the consistent thread is that serotonin production and clearance in the gut are disrupted in most functional digestive disorders.