Past studies have found that gut activity can have significant impacts on the brain, and vice versa.
Now, new research in mice explains how some of that communication might occur: through very small numbers of live bacteria traveling from the gut to the brain via the vagus nerve.
Researchers from Emory University in the US used a variety of mouse models designed to trigger ‘leaky gut’ conditions that might allow bacteria to seep into wider circulation – including mice put on high-fat diets designed to trigger atherosclerosis and genetically engineered to have Alzheimer’s and Parkinson’s-like diseases.
This kind of gut damage can lead to bacteria seeping out into the bloodstream and causing infection, but here the researchers found a very small amount of bacteria matching the gut microbes in the brain, without being detected in the blood.
The bacteria were found in the brain, whether the gut leakage was prompted by diet or models of disease.
And the blood didn’t appear to be the main method of travel: Further tests showed that when blocks were put on the vagus nerve, the major nervous system route from the gut to the brain, the presence of bacteria in the brain was significantly reduced, suggesting this nerve may act as a route.
“This data reveals a bacterial translocation axis from the gut to the brain, impacted by environmental (diet) and genetic factors, and warrants further investigation to determine if this phenomenon also occurs in humans,” write the researchers in their published paper.
The idea that bacteria can get into the brain from the gut via the nervous system is not completely new, but this study provides some of the clearest experimental evidence so far in mice.
Traces of bacteria have previously been found in the brains of people who died with Alzheimer’s, for example – though that’s not the same as seeing the live changes in living animals that the researchers observed here.
When certain gut microbes were transferred to mice, scientists later detected the bacteria in the gut and brain, but nowhere else in the body.
However, there are caveats to be aware of, besides the fact that this was a study of mice and not humans. The number of bacteria that made it to the brains of the mice was extremely low, and it’s not clear yet if it would be enough to trigger inflammation or disease.
This is something that future research can look into. Multiple past studies have found connections between gut inflammation and neurodegenerative diseases such as Alzheimer’s, as well as mood disorders, though causal links remain unclear.
Further down the line, it might even be possible to treat certain brain diseases and mental health issues with drugs aimed at changing the gut microbiome, though a clear cause-and-effect link would need to be established first.
“One of the biggest translational aspects of this study is that it suggests that the development of neurological conditions may be initiated in the gut,” says microbiologist David Weiss.
“This may shift the focus of new interventions for brain conditions, with the gut as the new target of the therapy. That potential anatomical shift of the target could have an unbelievable impact on how people with neurological conditions benefit from therapies.”
Scientists are currently investigating a host of different methods through which the gut and the brain might be talking to each other, including through the immune system, through the nervous system, and through multiple biochemical reactions. This new study suggests that the connection may be more direct, with bacteria taking a trip from one site to another under specific conditions.
One positive finding from the study is that when mice on a high-fat diet went back to eating normally, the bacteria appeared to drop below detectable levels in the brain – suggesting that whatever damage is done here could also be reversed by fixing the leakiness of the gut.
Related: Scientists Link Gut Bacteria to Bipolar Depression in Mouse Study
It’s another reminder of how interconnected the different parts of our bodies are, and how health problems in one region can quickly have a detrimental effect on another. For the gut-brain axis, that seems to be especially true.
“This research highlights the need for further study into how dietary shifts have a huge influence on human behavior and neurological health,” says immunologist Arash Grakoui.
The research has been published in PLOS Biology.
