Bacteria is not usually found in most organs in the body though. Even more so in an organ as important as the brain that even our own immune system doesn't have access. The brain has its own cells to do that job. In order for bacteria to cross the blood brain barrier you need bacteria in the blood first, where they aren't normally found without getting under fire.
Having bacteria in the gut is more obvious and makes more sense.
The microflora would need to have gotten there somehow, something analogous to endosymbiosis. But unless they're somehow getting from brain-to-egg-to-brain that's hard to explain. As for the others, they're easier, all being along the surfaces. Even the gut is on the "outside" in that membranes need not be crossed in order to access it.
Although counterpoint: the same argument would apply to a fish so I guess it's not impossible. Just more surprising.
Thus, both the spirochetes causing Lyme disease and those causing syphilis have high chances of reaching even the brain, while traveling through the body.
Fortunately, for the bacteria employing other means of locomotion it is usually more difficult to pass through many of the internal body tissues, as long as those are intact.
There are pathways from the outside to the brain - most notably the nose and the eyes. The former have a known pathogen pathway (naegleria fowleri, a virtually 100% fatal parasite), the latter are actually an immunoprivileged site [1] and investigations are ongoing what the role of eyes is in the transmission of H5N1 bird flu.
[1] https://www.aao.org/eye-health/tips-prevention/eye-immune-pr...
[2] https://www.unmc.edu/healthsecurity/transmission/2024/06/04/...
Not saying it's impossible, just that the skepticism is warranted.
There is. The placenta and, oddly, prostate as well.
For the more internal organs, bacteria would have to travel across a skin of digestive tract barrier.
Unless they're present in the womb as the fetus starts growing, I guess.
I thought that was recently found to be false? Or at least it isn't the full picture. see for example: https://www.science.org/doi/10.1126/science.abo7649
At least one pathogen, Epstein-Barr virus, is known to inhabit B cells.
It probably has to do with the fact that, topologically speaking, the stomach (and the whole GI tract) is technically on the outside of our bodies.
I guess this review/writeup https://asm.org/articles/2020/april/how-pathogens-penetrate-... covers most of the known mechanisms for traveral of the BBB by pathogens.
Now, it could be the case that in a person with a compromised immune system AND a compromised blood brain barrier, that organisms would be able to live long enough to reach the brain. Once there, they would be mostly shielded from the immune system, except for brain glial cells (and I guess antibodies; usually the blood brain barrier stops most of them)
But why should this be taken a sign of a brain microbiome? I fail to see the connection you're implying.
>>> Yes, fish brains have a blood-brain barrier (BBB)
Anyway there's a pdf here that probably tells what kind of a BBB zebrafish have.
https://www.mdpi.com/1422-0067/22/22/12111
And the article itself says "... communities of bacteria thriving in salmon and trout brains. Many of the microbial species have special adaptations that allow them to survive in brain tissue, as well as techniques to cross the protective blood-brain barrier."
Not so long ago we thought fish or even human babies didn't feel pain, we take a lot of things for granted. https://hms.harvard.edu/news/long-life-early-pain
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.