Researchers reported on Mar. 15 that certain gut bacteria can travel to the brains of mice without entering the bloodstream, suggesting a possible route via the vagus nerve. The findings were published in PLOS Biology and provide new insights into how the gut and brain may be connected.
The study is significant because it offers evidence that specific gut microbes might reach the brain through pathways other than blood circulation, raising questions about their potential role in neurological health. This could have implications for understanding conditions such as Parkinson’s disease, autism spectrum disorder, and Alzheimer’s disease.
In their experiments, scientists fed mice a high-fat Paigen diet commonly used in research on atherosclerosis. They found that while total bacterial counts in the intestines remained similar between test and control groups, PD-fed mice showed changes in their gut microbiome and increased intestinal permeability. Despite these changes, bacteria were not detected in most organs or blood but were found in small numbers within the brains of PD-fed mice. The species identified included Enterococcus faecalis, Staphylococcus sciuri, and Staphylococcus xylosus.
Further investigation revealed that these bacteria likely traveled along the vagus nerve rather than through systemic circulation. Mice that underwent vagotomy had significantly fewer bacteria present in their brains compared to those with intact nerves. Genomic analysis confirmed that bacteria found in the brain matched those from each mouse's own gut.
Additional tests showed that altering the gut microbiome with antibiotics changed which bacterial species appeared in the brain. When researchers introduced Enterobacter cloacae into antibiotic-treated or germ-free mice fed a high-fat diet, this bacterium was later detected both in their intestines and brains.
The authors emphasized that although low levels of culturable bacteria were also found in mouse models of neurodegenerative diseases maintained on standard diets, there is no evidence yet that this translocation causes such disorders. Extensive contamination controls were implemented due to very low microbial biomass detected.
The study concludes that certain gut bacteria can move to the brain via non-bloodstream routes like the vagus nerve under specific dietary conditions. Further research will be needed to determine if similar mechanisms occur in humans.
