Introducing the brain: does it have a microbiome? (Introduction)

by David Turell , Monday, December 02, 2024, 18:22 (1 day, 13 hours, 24 min. ago) @ David Turell

Bacteria are definitely in fish brains:

https://www.quantamagazine.org/fish-have-a-brain-microbiome-could-humans-have-one-too-2...

"...scientists have long assumed that bacteria can’t survive in the human brain. The powerful blood-brain barrier, the thinking goes, keeps the organ mostly free from outside invaders. But are we sure that a healthy human brain doesn’t have a microbiome of its own?

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"Recently, a study published in Science Advances provided the strongest evidence yet(opens a new tab) that a brain microbiome can and does exist in healthy vertebrates — fish, specifically. Researchers at the University of New Mexico discovered 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.

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"The human gut microbiome plays a critical role in the body, communicating with the brain and maintaining the immune system through the gut-brain axis. So it isn’t totally far-fetched to suggest that microbes could play an even larger role in our neurobiology.

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"To cover their bases, Salinas’ team studied the fishes’ whole-body microbiomes, too. They sampled the rest of the fishes’ brains, guts and blood; they even drained blood from the many capillaries of the brain to make sure that any bacteria they discovered resided in the brain tissue itself.

“'We had to go back and redo [the experiments] many, many times just to be sure,” Salinas said. The project took five years — but even in the early days it was clear that the fish brains weren’t barren.

"As Salinas expected, the olfactory bulb hosted some bacteria. But she was shocked to see that the rest of the brain had even more. “I thought the other parts of the brain wouldn’t have bacteria,” she said. “But it turned out that my hypothesis was wrong.” The fish brains hosted so much that it took only a few minutes to locate bacterial cells under a microscope. As an additional step, her team confirmed that the microbes were actively living in the brain; they weren’t dormant or dead.

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"Researchers have long been skeptical that the brain could have a microbiome because all vertebrates, including fish, have a blood-brain barrier. These blood vessels and surrounding brain cells are fortified to serve as gatekeepers that allow only some molecules in and out of the brain and keep invaders, especially larger ones like bacteria, out. So Salinas naturally wondered how the brains in her study had been colonized.

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"Salinas was able to identify features that let bacteria make the crossing. Some could produce molecules, known as polyamines, that can open and close junctions, which are like little doors in the barrier that allow molecules to pass through. Others could produce molecules that help them evade the body’s immune response or compete with other bacteria.

"Salinas even caught a bacterium in the act. Looking under the microscope, she captured an image of a bacterium frozen in time within the blood-brain barrier. “We literally caught it right in the middle of crossing,” she said.

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"...It’s possible that the microbes actively regulate aspects of the creatures’ physiology, Salinas suggested, the way human gut microbiomes help regulate the digestive and immune systems.

"Fish, of course, are not humans, but they allow a fair comparison, Salinas said. And her work suggests that if fish have microbes living in their brains, it’s possible we have them, too.

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"...in 2013, when scientists studying the neurological impacts of HIV/AIDS found genetic hints of bacteria in the brains of both sick and healthy people. The findings were the first to suggest(opens a new tab) that maybe humans could have a brain microbiome in the absence of disease.

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"...a human brain microbiome is not impossible. What is nearly impossible, however, is confirming that without harming healthy people. To build a case, Link suggested repeating the fish experiment in rodents. “This protocol should be able to be adapted really easily to mouse brains,” Salinas said — and indeed her team has started looking into it. They have found early hints that microbes exist in the olfactory bulbs of healthy mice and, to a lesser extent, throughout the brain.

“'There’s no reason, if fish have them, that you wouldn’t have them, or that mice wouldn’t have them,” Link said. If microbes have adapted to cross the fish blood-brain barrier and survive in the fish brain, they could do the same in our bodies. It’s unlikely they would be present at the same levels as they are in fish, he added, “but that doesn’t mean there’s none.'”

Comment: bacteria are everywhere supporting living organisms in gut biomes. Yes, as discussed in theodicy threads, there are bad actors who cause trouble in the wrong places. It is an equal result to having evil people with free will. We may find that a brain biome is as important as the gut biome.