Inside the human intestine is an ecosystem of microbes that live symbiotically with us and play a role in digestion. Other parts of the body, such as the skin, mouth, nasal cavity, and scalp, were also found to have their own unique microbiomes. Because of this, experts propose that the human brain could also have its own microbiome.
Blood-Brain Barrier
The concept of brain microbiome was first proposed in 2013 in the paper "Brain Microbial Populations in HIV/AIDS: α-Proteobacteria Predominate Independent of Host Immune Status." However, it did not get much attention due to the belief that the brain is a sterile organ that is protected from the rest of the body and from harmful agents that circulate in our bloodstream.
Aside from this, it is also difficult to confirm the presence of microbes since the techniques depend on the analysis of foreign genetic material. These approaches can be unreliable since the fragments of DNA can be the result of contamination.
In healthy human brains, there is a "blood-brain barrier" which protects the brain from the blood and other dangerous substances dissolved in it. However, this protective barrier becomes leaky when a person ages or when they experience neurological diseases like Alzheimer's disease. As a result, harmful substances can make their way into a person's brain.
Such an event can cause diseases and worsen the damage that has already taken place. As the immune system also becomes less effective when a person ages, the body can also be invaded by microorganisms that have been cleared away by immune cells in younger individuals.
Bacterial Invasion
In a 2013 study, a team of researchers investigated whether microbes can invade the brains of people who suffer from HIV/AIDS. Led by William G. Branton, the scientists compared the brain tissue of people with HIV/AIDS to the brain tissue of those without.
They discovered non-human genetic material, which pointed to the presence of over 173 types of bacteria and viruses in the brains they studied. All the tested brain samples from patients with various brain disorders appeared to contain bacterial genetic material.
In a recent study, a group of experts from the University of Edinburgh conducted a similar study where they compared the brains of people with Alzheimer's disease to healthy brains. It was found that the brains of people with Alzheimer's contain more bacteria and fungi than those of healthy people. Still, several species of bacteria, fungi and other microorganisms were found in healthy brains.
The microbiome in the human brain was also found to be a subset of the gut microbiome. While more bacteria were discovered in the brains of individuals with Alzheimer's, the scientists failed to detect a pattern of certain bacteria that were only found in diseased brains.
The researchers were also mystified as to how the microbes can make their way into the brain. One theory suggests that oral diseases, such as gum disease or tooth decay, cause tissue damage, which enables the bacteria normally found in the mouth to travel to the brain through the nervous system.
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