Can People Have a Brain Microbiome?

The human gut microbiome plays an important role in the body, communicating with the brain and maintaining the immune system through gut-brain axis. So it is not an exaggeration to say that viruses can play a major role in our neurobiology.

Fishing for insects

For years, Irene Salinas has been interested in a simple fact about the body: The distance between the nose and the brain is very small. The immunologist, who works at the University of New Mexico, studies the mucosal defense system in fish to understand how human systems, such as our intestines and nose, work. The nose, he knows, is full of bacteria, and is “really close” to the brain—just millimeters from the bulb, which produces odors. Salinas has always been of the opinion that bacteria can travel from the nose into the olfactory bulb. After years of curiosity, he decided to question his favorite species: 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. (A major contribution to the research was made by Amir Mani, who is the lead author of the paper.) They planned to look at the DNA sequence in the archive to identify each species.

These types of samples, however, are easily contaminated by bacteria in the lab or other parts of the fish’s body, which is why scientists struggle to properly study this issue. If they found bacterial DNA in the olfactory bulb, they would have to prove to themselves and other researchers that it actually came from the brain.

To cover their bases, Salinas’ team also studied the microbiomes of fish species. They tested the brain, intestines, and other blood of the fish; they bleed many parts of the brain to make sure that any bacteria they find live in the same brain.

“We had to go back and redo (the test) several times to make 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 scent bulb contained some bacteria. But he was surprised to see that his brain had many other things. He said: “I thought that some parts of the brain don’t have bacteria.” “But it turns out my assumptions were wrong.” The brain of the fish was so powerful that it only took a few minutes to find the bacterial cells using a microscope. As a further step, his team confirmed that the virus resides in the brain; they were not sleeping or dead.

Olm was impressed with their method. Salinas and his team circled “the same question, from all these different approaches, using all these different methods—all of which produced a convincing finding that there are pathogens in the fish brain,” he said.

But if there is, how did they get there?

Attacking the fort

Researchers have long suspected that the brain might have a microbiome because all living things, including fish, have it the blood-brain barrier. The blood vessels and cells surrounding the brain are encouraged to act as gatekeepers that allow certain molecules to enter and leave the brain and keep out invaders, especially large ones like bacteria. So Salinas naturally wondered how the brain in his research was set up.

By comparing the DNA of pathogens from the brain to those taken from other organs, his laboratory found organisms that were not found anywhere else in the body. Salinas thought that these species may have interfered with the fish’s brain early in its development, before the blood barriers were established. “Soon, anything can go in; it is free for all,” he said.