There are more than 100 million neurons in the gastrointestinal tract, and in many cultures, including ancient Greece, Japan, China, and India, the gut is commonly known as the “second brain,” linking digestion to physical and mental health.
Now, new research from Emory University explains the connection between the gut and the brain, showing that live bacteria from the gut can directly invade the brain, with potential effects on neurological health.
Published in PLOS Biology The study, published in March, was conducted in a mouse model and demonstrated that live bacteria from an imbalanced gut flora can enter the brain via the vagus nerve. Responsible for important functions such as heart rate and breathing, the vagus nerve connects the brainstem to major abdominal organs such as the heart, lungs, stomach, intestines, and liver.
One of the biggest translational aspects of this study is that it suggests that the development of neurological conditions may begin in the gut. ”
Dr. David Weiss, Co-Principal Investigator of this study
“This could make the gut a new target for therapy and shift the focus of new interventions in brain diseases. Changes in that potential anatomical target could have an incredible impact on how people with neurological diseases benefit from treatment,” added Weiss, a microbiologist and professor at Emory University School of Medicine.
In the study, a group of germ-free mice were fed a “paigen diet,” similar to a Western diet, containing 45% carbohydrates and 35% fat for nine days. These diets in humans are known to cause intestinal permeability, or “leaky gut” where compounds leak from the intestines.
As a result, the changes in the gut microbiota observed in mice were associated with increased permeability or leakiness of the intestinal barrier, allowing live bacteria to travel from the intestine directly to the brain via the vagus nerve without detectable amounts of bacteria in the blood or other organs.
To strengthen this concept, the researchers gave these mice antibiotics for three days that killed many gut bacteria. The mice then ingested a barcoded genetically engineered bacterium called Enterobacter cloacae that had DNA sequences not normally found in these bacteria in nature. When the mice were also fed a high-fat diet, this exact barcode strain was later detected in the vagus nerve and brain of the mice.
The study also highlights that strict methods were used to prevent cross-contamination, and that the bacterial load in the brain was low, within a few hundred cells, ruling out sepsis or meningitis.
Researchers also identified low levels of the bacteria in the brains of mouse models of neurological diseases such as Parkinson’s and Alzheimer’s disease, which could explain how these diseases develop in humans.
“This study highlights the need for further research into how changes in diet can profoundly impact human behavior and neurological health,” said co-principal investigator of the study Arash Grakowi, Ph.D., professor of medicine, microbiology, and immunology at Emory University.
Grakoui also noted that returning these mice to a normal diet reduced intestinal permeability and limited the amount of bacteria in the brain, indicating that the effects of a high-fat diet on bacteria reaching the brain may be reversible.
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Reference magazines:
Thapa, M. Others. (2026) Bacterial translocation from the gut to the brain in mice. PLOS Biology. DOI: 10.1371/journal.pbio.3003652. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3003652
