Traditionally, the brain has been considered sterile, meaning that it is free of bacteria, viruses, and other microorganisms.
Recent studies have challenged this notion and suggested that microbes such as bacteria and viruses may be present in the brain.
The glymphatic system, a pathway in the brain that allows for the clearance of waste products and the movement of immune cells throughout the brain, has been shown to be involved in the removal of microbial products, suggesting that microbes may be present in the brain.
In addition, studies have shown the presence of bacteria and other microorganisms in the gut and their potential effects on brain function and behavior, suggesting that there may be a gut-brain axis that could involve microbes in the brain.
Small subunit ribosomal RNA (rRNA) probes, confirmed by extensive subunit rRNA analysis find a spectrum of microorganisms in control and AD brains.
There are diverse brain microbes in control brains, the most abundant being fungi, bacteria, and chloroplastida.
The human brain microbiome appears to be a subset of the gut microbiome.
The spectrum of brain microbes differ between individuals and different brain regions, with the highest microbial burden found in the cingulate cortex.
Adenovirus type C is the primary virus in the human brain.
Spectrum of brain microbes differed between individuals and between brain regions examined from single individuals amygdala, cingulate cortex, hippocampus, hypothalamus) of these four regions, the highest microbial burden was in the cingulate cortex.
Certain microbes are more prevalent in the brains of individuals with Alzheimer’s disease (AD): Bacteria, including Streptococcus, Staphylococcus/Bacillus, and Sphingomonas/Ralstonia, as well as fungi such as Acrocalymma/Altenaria/Aureobasidium of the Aspergillus group, Komagataella of the Candida group, Cortinarius of the Schizophyllum group, and Tausonia of the Cryptococcus group, are found to be over-represented in AD brain.
Chloroplastida species (algae-related) are more abundant in AD brain samples.
These findings suggest a potential link between specific microbes and the development of AD.