When the intestinal barrier is weakened, live bacteria from the digestive system can directly enter the brain. The findings offer a new possible explanation for how digestive health influences neurological conditions such as Alzheimer’s disease and autism. This study was recently published in the journal PLOS Biology.
The gastrointestinal tract and central nervous system are closely connected through a biological communication network called the gut-brain axis. This network helps regulate bodily functions, digestion, and inflammation. Medical experts have noted the connection between the gut microbiome and various neurological conditions.
The gut microbiome is the vast collection of bacteria and other microorganisms that naturally live within the digestive tract. Changes in the types of bacteria that live in the gut often occur at the same time as a condition called intestinal permeability. This condition occurs when the lining of the intestines weakens and substances leak into the body.
High-fat diets are known to alter the bacterial composition of the intestines and contribute to this leaky gut. But researchers didn’t fully understand the exact pathways by which gut bacteria can directly affect the brain and cause neurological diseases. Manoj Thapa, a researcher at Emory University’s Emory National Primate Research Center, led a study exploring these physical pathways.
Thapa and his team of colleagues set out to find out whether microbes can physically move directly from the digestive system to the brain. To test their idea, the researchers used a special breed of lab mice that are prone to developing liver problems and changes in their gut bacteria. They fed these mice a high-fat, high-carbohydrate diet called the Pygen diet for nine days.
The research team then analyzed the mice’s feces and intestinal tissue. They observed that a high-fat diet changes the bacterial composition in the gut, increasing bacteria such as Staphylococcus and decreasing beneficial bacteria such as Lactobacillus. In addition to these bacterial changes, the high-fat diet weakened the mice’s intestinal lining, causing it to leak.
To determine whether the bacteria had leaked from the gastrointestinal tract, researchers examined various organs, including the lungs, heart, kidneys, and blood. They detected no bacteria in the blood or most other systemic organs. However, they found that mice fed a high-fat diet had small numbers of live bacteria in their brains.
The researchers then used genetic sequencing to compare the bacteria found in the brain to the bacteria found in the gut. They found that the genetic codes were an almost perfect match, indicating that the bacteria in the brain originated from the gut. Since no bacteria were found in the blood, the researchers needed to find another route for the microbes to reach the brain.
They focused on the vagus nerve, a long nerve pathway that connects the brain stem to the heart, lungs, and digestive organs. When researchers examined the cervical branch of the vagus nerve in mice, they found exactly the same type of bacteria. To test whether this nerve was a physical pathway, the researchers performed surgery to cut the right cervical vagus nerve in some mice.
Cutting nerves on both sides of the body would be fatal, so only one side was amputated. These surgically modified mice had significantly lower levels of bacteria in their brains compared to mice with intact nerves. So the researchers wanted to know whether the type of bacteria in the gut determines what ultimately reaches the brain.
They gave a new group of mice a blend of common antibiotics to kill off existing gut bacteria. They then introduced specific genetically modified strains of Enterobacter bacteria into the gastrointestinal tracts of these mice. This genetically modified strain contained a unique DNA barcode not found in nature.
After feeding these mice a high-fat diet to induce leaky gut, the researchers searched their brain tissue for this specific bacterial DNA. They were able to detect unique DNA barcodes in brain tissue using sensitive laboratory techniques that copy and amplify genetic material. This proved that certain bacteria placed in the gut traveled directly to the brain.
To confirm that the blood was indeed bacteria-free, researchers tested the blood for certain antimicrobial proteins. These proteins naturally surge when the immune system detects an infection in the bloodstream. Levels of these proteins remained completely normal, providing further evidence that the microorganisms do not use the circulatory system to move.
To ensure that these results were not limited to a specific mouse breed, they repeated the experiment using standard laboratory mice. When standard mice ate a high-fat diet, they developed leaky gut and harbored gut bacteria in their brains. The researchers observed that the bacteria appeared in the vagus nerve before appearing in the brain, supporting the idea of ​​a transit route.
The researchers also tested whether this physical movement of the bacteria was a permanent condition. The researchers took mice that had been eating a high-fat diet and put them back on a standard laboratory diet. After returning to a normal diet, the mice’s intestinal lining healed and the leakage stopped.
Afterwards, the researchers could no longer detect bacteria in the brains of these mice. This showed that the presence of bacteria in the brain is a reversible condition caused by the health of the intestinal lining.
The team then expanded their scope to investigate mice that were engineered to mimic human neurological conditions. They tested mouse models designed to mimic Alzheimer’s disease, Parkinson’s disease, and autism spectrum disorders. Even when eating a normal diet, these particular mice showed a weakened intestinal lining.
When researchers examined the brains and vagus nerves of these diseased mice, they found that intestinal bacteria were present. Similar to the dietary experiment, the bloodstream of these mice was free of bacteria. The blood-brain barrier, the protective filter that separates the brain from the circulatory system, remained completely intact in all animals tested.
This supported the idea that the microbes bypassed the bloodstream entirely and used the nerves as a highway. This result suggested that leaky gut may be a common feature that allows bacterial movement in these specific neurological conditions. The researchers took great care to avoid contaminating the samples during the collection process.
They did all of their work in a sterile environment and collected brain tissue before touching the digestive tract. They also confirmed that microbes were absent from the brains of germ-free mice kept in sterile bubbles free of natural bacteria. When the researchers fed these germ-free mice a single strain of bacteria and regular food, the microbes remained in their guts.
The bacteria only reached the brain when germ-free mice were fed a high-fat diet, which causes leaky gut. This proved that their isolation method was clean and that weakening of the intestinal lining was absolutely necessary for the bacteria to relocate. To fully understand the connection between leaky gut and the brain, the research team also gave mice a chemical that actively destroys the lining of the intestines.
Only when this chemical was administered at the absolute highest doses did bacteria eventually flood into the bloodstream. This showed that the moderate leaky gut caused by a high-fat diet was enough to send bacteria into the nerves, but not severe enough to cause a full-blown blood infection.
This study has several limitations that warrant further investigation by the scientific community. Because this study relied entirely on animal models, it remains unclear whether this precise physical transfer of bacteria occurs in humans. The number of bacterial cells found in brain tissue was very small, typically in the hundreds.
Although the differences in the exact amount of bacteria found in the brain between different mouse models were not statistically significant, the presence of bacteria was consistent. Researchers have not yet been able to capture visual images of bacteria within the brain or vagus nerve. The special diet used to induce leaky gut in mice is an extreme formula containing high levels of fat and certain acids.
Although this diet is different from typical human eating habits, the Western diet can also cause intestinal problems. It is not yet clear exactly where the bacteria live after reaching the brain. Scientists also need to determine which specific brain cells come into contact with these translocated microbes.
Future research will focus on whether all types of gut bacteria can travel along the vagus nerve, or whether only certain species can. The researchers will also investigate how long the bacteria can survive in the brain after the intestinal lining has healed. Understanding these pathways may ultimately lead to new treatments.
If doctors can target the digestive system and prevent bacteria from escaping, they may be able to change the course of some neurological conditions. “One of the biggest translational aspects of this study is that it suggests that the onset of neurological symptoms may begin in the gut,” said David S. Weiss, corresponding author of the study. “This could make the gut a new target for treatment and shift the focus of new interventions for brain diseases,” he said.
The study, “Transfer of bacteria from the gut to the brain in mice,” was conducted by Manoj Thapa, Anuradha Kumari, Chui-Yoke Chin, Jacob E. Choby, Elahe Akbari, Bikash Bogati, Fengzhi Jin, Elise Fur, Daniel M. Chopyk, Nitya Koduri, Andrew Pahnke, Theodore L. Burns, Elizabeth J. Elrod, Eileen M. Bird, David S. Weiss, Arash Gulakui.
