Could humans have a brain microbiome?

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

Fishing for microbes

For years, Irene Salinas he was fascinated by a simple physiological fact: the distance between the nose and the brain is quite small. An evolutionary immunologist 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 lining and nasal cavity, work. He knows that the nose is full of bacteria and that they are “really close” to the brain – just millimeters from the olfactory bulb, which processes smells. Salinas always believed that bacteria seeped into the olfactory bulb from the nose. After years of wondering, he decided to face his suspicions in his favorite model organisms: fish.

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

However, such samples are easily contaminated by bacteria in the lab or other parts of the fish’s body, so scientists struggle 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 actually originates in the brain.

To cover their bases, Salinas’ team also studied the fish’s whole-body microbiomes. They took the rest of the fish’s brains, guts, and blood; they even took blood from many of the brain’s capillaries to make sure that any bacteria they discovered had taken up residence in the brain tissue itself.

“We had to go back and redo (the experiments) many times to make 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 expected, the olfactory bulb contained some bacteria. But he was shocked to find that there was more in the rest of the brain. “I thought other parts of the brain wouldn’t have the bacteria,” he said. “But my assumption turned out to be wrong.” The fish’s brain took up so much space that it only took a few minutes to find the bacterial cells under a microscope. As a further step, his team confirmed that microbes actively reside in the brain; they were not asleep or dead.

Olm was impressed by their comprehensive approach. Salinas and his team “circled around the same question using all these different methods — all of which provided compelling evidence that there were indeed living microbes in the salmon’s brain,” Salinas said.

And if so, how did they get there?

The occupation of the castle

Researchers have long been skeptical that the brain might have a microbiome because all vertebrates, including fish blood-brain barrier. These blood vessels and the surrounding brain cells are reinforced to serve as gatekeepers, allowing only certain molecules to pass in and out of the brain and keeping out invaders, especially larger ones like bacteria. So Salinas was naturally interested in how the brains in his work were colonized.

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