Could the human brain have a microbiome?

The human gut microbiome plays an important role in the body, communicating with the brain and thereby maintaining the immune system The gut-brain axis. So it’s not far-fetched to suggest that microbes may play an even bigger role in our neurobiology.

Fishing for germs

over the years, Irene Salinas Fascinated by a simple anatomical 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 the human versions of these systems, such as our gut lining and nasal cavity, work. The nose, he knows, is loaded with bacteria, and they’re “really, really close” to the brain—just millimeters from the olfactory bulb, which processes odors. Salinas always had the idea that bacteria could leak from the nose to the olfactory bulb. After years of curiosity, he decided to confront his doubts in his favorite model organism: the fish.

Salinas and his team began by extracting DNA from the olfactory bulbs of trout and salmon, some caught in the wild and some grown in his lab. (Important contributions to the research were made by Amir Mani, the paper’s lead author.) They planned to look up DNA sequences in a database to identify any microbial species.

Such samples, however, are easily contaminated—by bacteria in the lab or from other parts of the fish’s body—which is why scientists have struggled to study the subject effectively. If they find bacterial DNA in the olfactory bulb, they will have to convince themselves and other researchers that it really originated in the brain.

To cover their bases, Salinas’ team also studied the fish’s whole-body microbiomes. They took brain, gut and blood samples from the rest of the fish; They even drew blood from many of the brain’s capillaries to confirm that the bacteria they discovered were in the brain tissue itself.

“We had to go back and do (the tests) again, many times to be sure,” Salinas said. The project took five years—but even in the early days it was clear that the fish’s brain was not sterile.

As Salinas expected, the olfactory bulb hosts some bacteria. But he was shocked to find that the rest of the brain was even more. “I thought other parts of the brain wouldn’t have bacteria,” he said. “But it turns out my guess was wrong.” The fish’s brain hosted so many that it only took a few minutes to identify bacterial cells under a microscope. As an additional step, his team confirmed that the microbes were actively living in the brain; They were not dormant or dead.

Olm was impressed by their thorough approach. Salinas and his team “rounded the same question, from all these different ways, using all these different methods—all of which produced convincing data that there are actually living microbes in the salmon brain,” he said.

But if there is, how did they get there?

attack the castle

Researchers have long suspected that the brain might have a microbiome as all vertebrates, including fish, have A blood-brain barrier. These blood vessels and surrounding brain cells are protected to act as gatekeepers that allow certain molecules in and out of the brain and keep out invaders, especially large ones like bacteria. So Salinas naturally wondered how brains were colonized in his research.

By comparing microbial DNA from the brain to that collected from other organs, his lab found a subset of species not found elsewhere in the body. Salinas hypothesized that these species colonized the fish’s brain early in their development, before their blood-brain barrier was fully formed. “Primarily, anything can enter; It’s free for all,” he said.