Agreed. Additionally, one of the mechanisms keeping the gut bacteria from taking over the rest of the body is an oxygen gradient: anaerobic bacteria in the gut are unlikely to be interested in an exploration of the aerobic environment of the body. Oxygen diffuses freely across the blood brain barrier, so it would not serve as an analogous containment mechanism.

The lungs, mammary glands, uterus, ovaries, vagina, placenta, semen, eyes, skin, and nasal cavity all have their own documented microbiome so far, and we've been finding out about new microbiomes about once a year. They are not a thing that is reserved for the digestive tract.

If you were to simplify the human body to a sphere (or torus... or whatever corresponds with the number of orifices that a human has), then all of the microbiomes you've listed would be on the exterior. The brain (and heart and kidneys and liver and bones and...) would not be.

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.

I think it depends a bit on how you count but I've heard it is an 8 holed donut

It also differs between individuals (e.g. one can develop a fistula).

Or get a piercing.

So getting pierced, puts one topological outside of baseline humanity?

I suppose it does. Be careful, you wouldn't want to end up homeomorphic to the wrong crowd.

Aren’t tattoos injective?

When two artists do each other's backs, bijective.

If I met a 8 holed donut I'd tear it a new one.

I expect I'd start by tearing it one fewer: now a seven holed donut with some stubs.

Michael Stevens of VSauce fame has you covered with his "How Many Holes Does a Human Have?" video:

https://www.youtube.com/watch?v=egEraZP9yXQ

Thanks for mentioning this. I really enjoyed the video and learned a thing or two. Would recommend!

We already know of harmful pathogens that can infect the brain, such as Borreliosis. The only jump now is the fact that there could also exist harmless bacteria.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8232152/

Spirochetes are much more efficient than most bacteria at moving through very viscous media, which include the obstacles encountered when moving through an animal body.

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.

> But unless they're somehow getting from brain-to-egg-to-brain that's hard to explain.

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/...

The brain is _unusually_ protected though. There's nothing like the blood brain barrier basically anywhere else. Most the the tissues you mentioned are exposed to the environment in some way. There aren't many ways to reach the brain without getting inside the body first(sensory organs being the obvious pathway, but even that is a stretch).

Not saying it's impossible, just that the skepticism is warranted.

> There's nothing like the blood brain barrier basically anywhere else.

There is. The placenta and, oddly, prostate as well.

Those are all organs that are directly exposed to outside materials.

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.

Indeed, those organs expose to the outside world have reason to cultivate friendly bacteria for no other reason than to take up space, to stop hostile bacteria having a place to take hold.

> even our own immune system doesn't have access

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

It is certainly true as a rule of thumb, although there may be more going on locally that was previously thought

This is unrelated to your main point, more to clarify an edge case for those interested in learning more: B cells can breach the BBB under certain conditions.

At least one pathogen, Epstein-Barr virus, is known to inhabit B cells.

> Having bacteria in the gut is more obvious and makes more sense.

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.

But bacteria are incredibly adaptable

The gut is an involution of the outside surface; whatever is in the gut is essentially outside of the body, the same way that the inside of the hole of a donut is actually outside of the donut.