Could Humans Have a Brain Microbiome?

The human gut microbiome plays a vital role in the body, by communicating with the brain and maintaining the immune system gut-brain axis. So it’s not entirely far-fetched to suggest that microbes could play a greater role in our neurobiology.

fishing for microbes

Over the years, Erin Salinas I have always been fascinated by a simple physiological fact: the distance between the nose and the brain is quite small. The evolutionary immunologist, who works at the University of New Mexico, studies the mucosal immune systems of fish to better understand how human versions of these systems, such as our gut linings and nasal cavities, work. She knew that the nose was full of bacteria and that they were “very, very close” to the brain – just millimeters from the olfactory bulbs that process smells. Salinas had always had a hunch that bacteria might be leaking from the nose into the olfactory bulb. After years of curiosity, she decided to face her doubts using her favorite model organism: fish.

Salinas and her team began by extracting DNA from the olfactory bulbs of trout and salmon, some caught in the wild and some raised in her lab. (The paper’s lead author, Amir Mani, made a major contribution to the study.) They plan to search the database for DNA sequences that could identify any microbial species.

However, such samples can easily be contaminated by bacteria in the laboratory or from other parts of the fish’s body, which is why scientists have struggled to study this topic effectively. If they did find bacterial DNA in the olfactory bulb, they would have to convince themselves and other researchers that it did originate in the brain.

To cover their bases, Salinas’ team also looked at the fish’s whole-body microbiome. They sampled the rest of the fish’s brain, guts, and blood; they even drained the blood from the brain’s many capillaries to make sure any bacteria they found were present in the brain tissue itself.

“We have to go back and do it again [the experiments] Many, many times just to make sure,” Salinas said. The project took five years – but even in those early days it was clear that fish brains were not barren.

As Salinas expected, the olfactory bulb contained some bacteria. But she was surprised to find that there was more to the rest of the brain. “I don’t think there are bacteria in other parts of the brain,” she said. “But it turns out my hypothesis was wrong.” Fish brains contain so many bacteria that it only takes a few minutes to find bacterial cells under a microscope. As an extra step, her team confirmed that the microbes are actively living in the brain. They are not dormant or dead.

Orm was impressed by their thorough approach. Salinas and her team “approached the same question in all these different ways, using all these different methods — all of which produced compelling data showing that there are actually live microorganisms in salmon brains,” Salinas said. ” he said.

But if so, how did they get there?

Invasion of the Fortress

Researchers have long wondered whether there is a microbiome in the brain because all vertebrates, including fish, have microbiomes. blood brain barrier. These blood vessels and surrounding brain cells are hardened to act as gatekeepers, allowing only certain molecules in and out of the brain and blocking invaders, especially larger molecules like bacteria. So Salinas naturally wanted to know how the brains in her study were colonized.

By comparing microbial DNA in the brain with microbial DNA collected from other organs, her lab discovered a subset of species not found elsewhere in the body. Salinas speculates that these species may have colonized the fish’s brain early in its development, before the blood-brain barrier is fully formed. “In the early days, anything could get in; it was a free-for-all,” she said.