Human brains, presumably mammal brains in general, do not have microbiomes. If they did, they would look like Donald Sutherland in that movie.
Also, a microbiome is not the same thing as an infection. A microbiome is a mutualistic (or similar) ecology of multi-celled organisms or part thereof (like, your gut or your eyeballs or something) and microbes, probably including multiple species or varieties. Brains do not have that. If there are microbes in the brain it is an infection.
There is some interesting research out there possibly linking infections and Alzheimers. It is unfortunately being couched in terms of microbiomes. Why? Mainly because science reporters are generally not scientists, so they don’t bump on errors like that? Maybe. But in this case, there seems to be an actual project that claims to be actually mapping out the brain’s microbiome, including “helpful” organisms.
And here is a Twitter Feed confirming what I say above.
Compelling evidence for a human brain microbiome is non-existent (see also placenta microbiome) https://t.co/aFZc1hiQji
— Nick Loman (@pathogenomenick) August 5, 2018
If you have evidence to the contrary please post it below.