Do gut bacteria make neurotransmitters?

Yes. Certain gut bacteria produce or trigger neurotransmitters — for example Lactobacillus and Bifidobacterium make GABA, and gut cells make over 90% of the body’s serotonin. Most gut-made neurotransmitters don’t cross the blood–brain barrier directly; they act via the vagus nerve, precursors, and short-chain fatty acids.

The gut–brain axis

Neurotransmitters & your microbes

Your gut is a chemical factory — it makes over 90% of your serotonin — and specific bacteria help produce or trigger the brain’s key signalling molecules. One honest caveat: most of these don’t cross the blood–brain barrier directly. They act through the vagus nerve, the gut’s own hormone cells, precursors that DO cross, and short-chain fatty acids.

GABA

classical · γ-aminobutyric acid · calming

The main inhibitory (calming) neurotransmitter — the same target as many anti-anxiety drugs. Certain bacteria make it via the enzyme glutamate decarboxylase; Bifidobacterium adolescentis is a model producer, and its abundance correlates inversely with depression and anxiety.

Produced or stimulated by

Lactobacillus Bifidobacterium Bifidobacterium adolescentis

Glutamate

classical · the main excitatory signal

The brain’s primary excitatory neurotransmitter and the direct precursor to GABA. Many gut bacteria produce and interconvert it; the glutamate–GABA balance is a key lever the microbiome can nudge.

Produced or stimulated by

LactobacillusBacteroidesCorynebacterium glutamicum

Serotonin

classical · 5-HT · mood & gut motility

Over 90% of the body’s serotonin is made in the gut. Bacteria mostly stimulate the gut’s own enterochromaffin cells to produce it rather than making it directly — and it governs gut motility as much as mood.

Produced or stimulated by

Turicibacterspore-forming Clostridia

Dopamine

classical · reward & motor control

Some gut bacteria synthesize dopamine directly; more importantly, they supply and modify its precursors. Dopamine itself can’t cross the blood–brain barrier, so the gut’s influence travels by other routes.

Produced or stimulated by

Escherichia coliBacillus

Norepinephrine

classical · arousal & stress

This one runs conspicuously both ways: pathogens like E. coli and Salmonella SENSE host stress hormones (norepinephrine and adrenaline) as a growth signal — the bidirectional heart of the gut–brain axis, where your stress can literally feed certain microbes.

Produced or stimulated by

Escherichia coliBacillus

Acetylcholine

classical · memory & muscle

Central to memory and every muscle contraction. Produced by strains including Lactobacillus plantarum; it also carries much of the vagus-nerve signalling that connects gut and brain.

Produced or stimulated by

LactobacillusBacillus

Histamine

classical · immune & neuromodulator

Made by bacteria via histidine decarboxylase. It bridges immunity and the nervous system, and bacterial histamine production is central to histamine intolerance and gut inflammation.

Produced or stimulated by

LactobacillusMorganella morganiiEnterobacteriaceae

Tyramine

trace amine · from tyrosine

A trace amine bacteria make by decarboxylating tyrosine. It can raise blood pressure and is a classic dietary migraine trigger — a direct line from microbial metabolism to physiology.

Produced or stimulated by

EnterococcusLactobacillus

Tryptamine

trace amine · from tryptophan

Produced by gut bacteria from tryptophan. It speeds gut transit and modulates serotonin signalling — one of the clearest cases of a microbe-made molecule acting on the host nervous system.

Produced or stimulated by

Clostridium sporogenesRuminococcus gnavus

Short-chain fatty acids

metabolite · butyrate · propionate · acetate

The dominant chemical channel from gut to brain. Made when fibre-fermenters break down fibre, SCFAs feed the gut lining, calm microglia, tune the blood–brain barrier, and signal along the vagus nerve. Not a neurotransmitter in the textbook sense — but arguably the microbiome’s most important neuroactive output.

Produced or stimulated by

Faecalibacterium prausnitzii Roseburia

Indoles

metabolite · from tryptophan

Tryptophan-derived metabolites that act as signalling molecules (including as AhR ligands), shaping the gut barrier, immune tone, and behaviour in animal models.

Produced or stimulated by

Escherichia coliBacteroides

Hydrogen sulfide (H₂S)

gasotransmitter · double-edged gas

A gaseous signalling molecule from sulfate-reducing bacteria. At low levels it acts as a neuromodulator and protects the gut; in excess it damages the lining — a genuinely dose-dependent, double-edged signal.

Produced or stimulated by

DesulfovibrioBilophila wadsworthia

And it runs both ways. Under stress, your body releases catecholamines that certain pathogens sense as a growth signal — so your mental state can reshape the community that’s shaping your mental state. The gut–brain axis is a loop, not a one-way street.