I mentioned Adam Savage's "Scariest [Podcast] Episode Yet" in a different context the other day[0] in which I remember Joe DeRisi[1] saying something along the lines of brain & spinal fluid being "absolutely pristine" when it comes to the presence/absence of a microbiome (in healthy individuals) and that it'd be real problem if there were one.
I'm just a layman but can anyone ELI5 how this can be squared with the OP, specifically with statements like
> It turns out our grey matter is teeming with bacteria, viruses and fungi