What is the vagus nerve?
The vagus nerve is the longest cranial nerve, running from the brainstem down into the gut, heart, and other organs. It's the backbone of the gut-brain axis — and, notably, most of its fibers carry information upward, from gut to brain. That direction is what makes it a plausible route for bacteria in your intestine to influence how you feel.
The experiment that proved the connection
The clearest evidence comes from a 2011 study: mice given a Lactobacillus rhamnosus strain showed reduced stress hormones, calmer behavior, and changes in brain GABA receptors. When researchers cut the vagus nerve, all of those effects disappeared [1]. That's how the field established the vagus as a major route for the microbiome to talk to the brain — not correlation, but a severed-cable experiment.
What travels the wire
Gut bacteria signal the brain through several channels that converge on or run alongside the vagus: neurotransmitters they produce (like GABA), immune messengers, and metabolites. In social-behavior studies, Lactobacillus reuteri's effect on oxytocin also depended on an intact vagus nerve [2]. Different bacteria, same cable.
Why this matters for neurobiotics
Understanding the vagus nerve reframes what a neurobiotic is: not a mood pill, but an organism whose signals ride a real, traceable nerve to the brain. And because those signals are strain-specific, knowing exactly which strains are in your gut — and your formula — is the whole point. That's the map Flore builds with whole-genome sequencing. Read the full neurobiotics guide →
References
- Bravo JA, et al. Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. PNAS. 2011;108(38):16050–16055. pnas.org
- Sgritta M, Buffington SA, et al. Mechanisms underlying microbial-mediated changes in social behavior in mouse models of autism spectrum disorder (vagus-dependent Lactobacillus reuteri effect). ncbi.nlm.nih.gov