By Janosch Heller - Assistant Professor in Biomedical Sciences, Dublin City University
The microbes living in your gut have repeatedly made headlines in recent years. Even if you don't follow research in this area, chances are you haven't missed the numerous advertisements for probiotics, prebiotics, and other products designed to keep your gut microbiome healthy.
Other microbiomes have also been discovered recently, playing significant roles in our health. Our
mouth, our
nasal cavity, our
skin, and even our scalp all have their
own microbiomes. Some scientists have even suggested that the brain might have its own microbiome.
This idea, first proposed in
2013, initially didn't attract much attention. And for good reason: it was long believed that the brain was a
sterile organ, partly isolated from the rest of the body by the "blood-brain barrier," which protects it from potential harmful agents circulating in the blood.
Ten years later, where do we stand? Confirming the presence of microbes in the brain is difficult for various technical reasons, primarily because analyses highlighting the presence of foreign genetic material can sometimes be contaminated during sample processing. However, a body of evidence has been collected.
When the blood-brain barrier fails to do its job
Normally, the brain is protected from potential infectious agents circulating in the blood, as well as certain harmful substances that might be dissolved in it, by the blood-brain barrier. This very tight physiological barrier exists between the bloodstream and the central nervous system (brain and spinal cord).
However, as we age and in neurological diseases such as Alzheimer's, this protective barrier becomes permeable, allowing harmful substances to enter the brain. This situation can be the source of certain pathologies.
Moreover, as we age, the immune system also loses its efficiency. Consequently, microorganisms generally eliminated in younger individuals might persist in the bodies of older people.
In 2013, a team wanted to find out if microbes could invade the brains of people infected with the HIV virus and who had developed acquired immunodeficiency syndrome (AIDS). After obtaining consent from the patients, they compared (post-mortem, after autopsy) their brain tissue with that of non-infected individuals (including brain samples from people who had undergone surgery to treat epilepsy). To their surprise, they found numerous traces of non-human genetic material: over 173 types of bacteria and phages (viruses that infect bacteria) were detected in the studied brains.
Even more astonishing: all tested samples seemed to contain bacterial genetic material, not just those from HIV-infected individuals.
At the University of Edinburgh, another research team
compared the brains of people with Alzheimer's disease to healthy brains. While the brain samples of the latter housed more bacteria and fungi than those of healthy individuals, several species of fungi, bacteria, and other microorganisms were also found in healthy brains. Furthermore, even though more bacteria were present in the brains of Alzheimer's patients,
The analyses also revealed that the human brain microbiome appears to be a subset of the gut microbiome (sharing about 20% of the species). Although more bacteria were found in the brains of Alzheimer's patients, the researchers were not able to determine a specific pattern among the patients.
It should be noted that this study has not yet been peer-reviewed; it has only been posted as a "preprint" on the Bioarchive server, so these results should be interpreted with caution.
Questions remain unanswered
Ultimately, we still don't know how microorganisms manage to enter our brain.
One proposed theory suggests that health issues affecting the mouth, such as gum disease or tooth decay, could cause tissue damage, allowing
bacteria normally present in the oral cavity to travel to the brain via the nervous system.
Interestingly, oral bacteria can produce amyloid proteins. This family of proteins is also present in brain cells. Important for normal brain function, they can sometimes form abnormal clumps, notably observed in the brains of Alzheimer's patients.
With advances in molecular techniques, particularly new sequencing technologies, more microbes will likely be identified in places in the body where they had not been detected before. The notion of a brain microbiome is recent and captivating. As with the gut microbiome, an imbalance in the ratio of different microbial species present might be the source of diseases. If this were the case, this discovery could open up new therapeutic avenues, particularly in the context of certain brain diseases such as Alzheimer's.
However, various questions remain unanswered. We know that the gut microbiome varies from person to person, and the same might be true for the brain microbiome. We do not yet have a mapping of the microbes that might reside in the brain. Furthermore, we don't know how they are controlled or how they manage to enter the brain in the first place...