> Matthew Olm (opens a new tab), a physiologist who studies the human microbiome at the University of Colorado, Boulder and was not involved with the study, is “inherently skeptical” of the idea that populations of microbes could live in the brain, he said. But he found the new research convincing. “This is concrete evidence that brain microbiomes do exist in vertebrates,” he said. “And so the idea that humans have a brain microbiome is not outlandish.”

It's interesting to me that it would ever have been considered outlandish. In light of everything we now know about microbiomes and microbiology in general, it seems to me as a layman that the more radical proposition would be that the blood-brain barrier would be 100% effective at keeping out all bacteria, rather than the proposition that it probably isn't but that the bacteria it does let in tend to be symbiotic.