5 ms·

> 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, Bact

> 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, 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.

eleveriven 2y ago
But bacteria are incredibly adaptable
lolinder 2y ago
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.
- outworlder 2y ago
  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.
  - forgotoldacc 2y ago
    > There's nothing like the blood brain barrier basically anywhere else. There is. The placenta and, oddly, prostate as well.
- __MatrixMan__ 2y ago
  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.
  - Wilsoniumite 2y ago
    I think it depends a bit on how you count but I've heard it is an 8 holed donut
    - __MatrixMan__ 2y ago
      It also differs between individuals (e.g. one can develop a fistula).
      - eru 2y ago
        Or get a piercing.
        InDubioProRubio 2y ago
        So getting pierced, puts one topological outside of baseline humanity?
        __MatrixMan__ 2y ago
        I suppose it does. Be careful, you wouldn't want to end up homeomorphic to the wrong crowd.
- BurningFrog 2y ago
  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.
  - askvictor 2y ago
    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.
__MatrixMan__ 2y ago
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.
20after4 2y ago
> 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 https://www.science.org/doi/10.1126/science.abo7649
- s1artibartfast 2y ago
  It is certainly true as a rule of thumb, although there may be more going on locally that was previously thought
panabee 2y ago
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.
ghurtado 2y ago
> 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.
kazinator 2y ago
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.

eleveriven 2y ago

But bacteria are incredibly adaptable

lolinder 2y ago

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.

outworlder 2y ago
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.
- forgotoldacc 2y ago
  > There's nothing like the blood brain barrier basically anywhere else. There is. The placenta and, oddly, prostate as well.
__MatrixMan__ 2y ago
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.
- Wilsoniumite 2y ago
  I think it depends a bit on how you count but I've heard it is an 8 holed donut
  - __MatrixMan__ 2y ago
    It also differs between individuals (e.g. one can develop a fistula).
    - eru 2y ago
      Or get a piercing.
      - InDubioProRubio 2y ago
        So getting pierced, puts one topological outside of baseline humanity?
        __MatrixMan__ 2y ago
        I suppose it does. Be careful, you wouldn't want to end up homeomorphic to the wrong crowd.
BurningFrog 2y ago
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.
- askvictor 2y ago
  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.

__MatrixMan__ 2y ago

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.

20after4 2y ago

> 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 https://www.science.org/doi/10.1126/science.abo7649

s1artibartfast 2y ago
It is certainly true as a rule of thumb, although there may be more going on locally that was previously thought

panabee 2y ago

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.

ghurtado 2y ago

> 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.

kazinator 2y ago

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.