Now a new study suggests that our gut bacteria could even play a role in protecting us from brain damage, with an experiment involving mice showing that certain types of stomach microbes can actually help reduce the severity of strokes.
"Our experiment shows a new relationship between the brain and the intestine," said neuroscientist Josef Anrather from the Feil Family Brain and Mind Research Institute at Cornell University. "The intestinal microbiota shape stroke outcome, which will have an impact [on] how the medical community views stroke and defines stroke risk."
Anrather and his colleagues analyed two groups of mice – one received a combination of antibiotics that tweaked their gut microbiota, and the other acted as a control group, with no alterations made to their gut microbiota over the course of the experiment.
A fortnight after one group started receiving the antibiotics, the scientists induced an ischemic stroke in individuals from both groups. This is one of the most common forms of stroke, usually caused when an obstructed blood vessel prevents blood from reaching the brain.
In the experiment, the animals that had been treated with antibiotics beforehand experienced a stroke that was about 60 percent less severe than the control group.
While the researchers don't fully understand the mechanism, they suspect that the altered microbial environment in the treated animals' guts somehow directed immune cells to protect the brain by reducing the impact of inflammation and other destructive processes. This in turn shielded the brain from the full severity of the stroke experienced by the control group.
"One of the most surprising findings was that the immune system made strokes smaller by orchestrating the response from outside the brain, like a conductor who doesn't play an instrument himself but instructs the others, which ultimately creates music," said one of the team, Costantino Iadecola.
While there's no guarantee that the same kind of medical treatment would necessarily buffer humans from the impact of stroke, it's a promising line of enquiry for future research. One factor in particular that warrants further investigation is which bacterial components are leveraging the immune system to protect the brain against damage.
The findings, reported in Nature Medicine, suggest that the microbiota do not interact with the brain chemically, but somehow influence immune cells to promote neural survival. These immune cells end up making their way to the meninges – the outer covering of the brain – where they organise a response to mitigate the stroke.
If scientists can figure out just what's going on here – and if the same kind of brain-intestine relationship exists in people – it could lead to targeted microbiotic treatments to protect high-risk individuals from stroke, or even new approaches to dietary management that could provide a partial defence against brain damage.
"Dietary intervention is much easier to accomplish than drug use, and it could reach a broad base," said Anrather. "This is a little far off from the current study – it's music of the future. But diet has the biggest effect [on the] composition of microbiota, and once beneficial and deleterious species are identified, we can address them with dietary intervention."