Researchers from Emory University report in a March PLOS Biology paper that a short high-fat feeding can allow live gut bacteria to reach the brain in mice. The authors note the gut contains more than 100 million neurons and has long been called a "second brain" in cultures such as ancient Greece, Japan, China and India. The study used germ-free mice fed Paigen’s Diet, a Western-like regimen, for nine days; the diet contains 45% carbohydrate and 35% fat.
The high-fat feeding changed the gut microbiome and increased intestinal barrier permeability, commonly described as a "leaky gut." That leakage permitted live bacteria to travel from the intestine to the brain along the vagus nerve, the pathway that links the brainstem to the heart, lungs, stomach, intestines and other abdominal organs.
In one experiment the team treated mice with antibiotics for three days to reduce native microbes, then introduced an engineered, barcoded Enterobacter cloacae strain with a DNA sequence not normally found in nature. When those mice ate the high-fat diet, the barcoded strain was later detected in the vagus nerve and in the brain, while no bacteria were detectable in blood or other organs. Bacterial loads in the brains were low, within the hundreds, and strict procedures were used to avoid cross-contamination; the observed levels rule out sepsis or meningitis.
The investigators also observed low numbers of bacteria in brains of mouse models of Parkinson’s Disease and Alzheimer’s. Co-principal investigator David Weiss said the findings "suggest that the development of neurological conditions may be initiated in the gut." Co-principal investigator Arash Grakoui noted that returning mice to a normal diet reduced gut permeability and restricted bacterial load in the brain. The researchers call for further study of how dietary changes influence human neurological health and behaviour.
Difficult words
- microbiome — community of microbes living in one body area
- permeability — ability of tissue to let substances pass
- vagus nerve — nerve connecting the brainstem to abdominal organs
- barcode — mark an organism with a unique DNA tagbarcoded
- sepsis — dangerous whole-body reaction to severe infection
- meningitis — inflammation of the membranes around the brain
- germ-free — kept without any live microbes or germs
- regimen — planned pattern of diet or medical treatment
Tip: hover, focus or tap highlighted words in the article to see quick definitions while you read or listen.
Discussion questions
- What do these findings suggest about the possible link between diet and neurological conditions? Give reasons from the article.
- What additional experiments or controls would you propose to confirm that bacteria travel along the vagus nerve to the brain?
- Do you think similar bacterial movement from gut to brain could occur in humans? What challenges would researchers face in testing this?
Related articles
Protein sensor lets MRI detect molecular activity
Researchers developed a genetically encoded protein sensor, called MAPPER, that makes molecular activity visible to MRI by changing water movement in cells. The modular system could let scientists image processes like cancer and inflammation continuously in animals.