Can humans have a brain microbiome?

The human gut microbiome plays a crucial role in the body, communicating with the brain and maintaining the immune system through it Gut-brain axis. So, it is not entirely far-fetched to suggest that microbes could play a larger role in our neurobiology.

Hunting microbes

For years, Irene Salinas He was fascinated by a simple physiological fact: the distance between the nose and the brain is very small. The evolutionary immunologist, who works at the University of New Mexico, studies the mucosal immune systems in fish to better understand how human versions of these systems, such as the lining of the intestines and nasal cavities, work. She knows that the nose is full of bacteria, which are “very, very close” to the brain, just millimeters from the olfactory bulb, which processes smell. Salinas always had a hunch that bacteria might seep from the nose into the olfactory bulb. After years of curiosity, she decided to confront her doubts about her favorite model organisms: fish.

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

However, these types of samples are easily contaminated – by bacteria in the laboratory or from other parts of the fish’s body – which is why scientists struggle to study this topic effectively. If they find bacterial DNA in the olfactory bulb, they will have to convince themselves and other researchers that it actually originated in the brain.

To cover their bases, Salinas’ team studied the fish’s entire body microbiome as well. They took samples of the fish’s brains, entrails, and blood. They even drew blood from several capillaries in the brain to ensure that any bacteria they discovered were in the brain tissue itself.

“We had to go back and redo (the experiments) several times just to be sure,” Salinas said. The project took five years, but even in the early days it was clear that fish brains were not barren.

As Salinas predicted, the olfactory bulb contains some bacteria. But she was shocked to see that the rest of the brain contained more. “I thought other parts of the brain wouldn’t have bacteria,” she said. “But it turns out that my hypothesis was wrong.” The fish brains hosted so many organisms that it only took a few minutes to locate bacterial cells under a microscope. As an additional step, her team confirmed that microbes were actively living in the brain; They were not asleep or dead.

Ulm was impressed by their comprehensive approach. Salinas and her team said they answered “the same question, in all these different ways, and using all these different approaches — all of which produced convincing data that there are indeed living microbes in the salmon brain.”

But if there was, how did they get there?

Invasion of the castle

Researchers have long doubted whether the brain contains a microbiome, because all vertebrates, including fish, do Blood-brain barrier. These blood vessels and the brain cells surrounding them are barricaded to act as gatekeepers that allow only certain molecules in and out of the brain and prevent invaders, especially larger ones such as bacteria, from getting out. Naturally, Salinas wonders how brains are colonized in her study.

By comparing microbial DNA from the brain with DNA collected from other organs, her lab found a subset of species that did not appear anywhere else in the body. Salinas hypothesized that these species may have colonized the fish’s brains early in development, before their blood-brain barriers were fully formed. “Early on, anything can happen; it’s a free-for-all,” she said.