> 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.
> 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.
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.
> 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.
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.
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.
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.
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.
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.
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.
From what I understand about the BBB, it's practically impermeable to anything the size of a bacteria (mainly intended for gating small molecules like fats and drugs), until an individual has some sort of disease or other health problem that effects the BBB's integrity. But I can easily imagine that some bacteria (harmful, or helpful) could have exactly the right surface proteins to indicate it's permitted.
CSF sampling is a routine investigation in most hospitals. We don't typically see bacteria in these samples, when we do, its in the form of bacterial meningitis, hence the skepticism.
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."
Is it the blood brain barrier that keeps out bacteria or your immune system? Admittedly I know little about biology, but I thought bacteria growing inside your body was generally considered a “bad thing” (topologically speaking, your digestive system is outside your body).
Indeed. In order to even try to cross the blood brain barrier, an organism would have to be in the blood already. If that's unexpected, the immune system certainly won't like it. Large compounds and even immune cells aren't supposed to cross it. Usually brain infections are from tiny viruses because of that, unless they found another way (like ear, nose or eye infections, all of them effectively brain 'appendages').
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)
I like to think that the bacteria are only on the outermost layer of the body, i.e. skin. It just happens so that the respiratory and the digestive systems are the exterior part of the body which are inside our body. In other words epithelial cells mark the boundary of inside and outside and bacteria are usually tolerated on the outside and not inside.
Pure intuition, nothing more. I think we're just beginning to understand the microbiome and the brain does so much, has so many ports, and requires so many resources, that I think there are probably hundreds or thousands of heretofore unknown critters it relies on.
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.