Could humans have a brain microbiome?

The human gut microbiome plays a crucial role in the body by communicating with the brain and maintaining the immune system the gut-brain axis. So it’s not entirely unreasonable to assume that microbes could play an even larger role in our neurobiology.

Fishing for microbes

for years, Irene Salinas was 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 system of fish to better understand how human versions of these systems, such as our intestinal lining and nasal cavity, work. She knows that the nose is full of bacteria and that they are “very, very close” to the brain – just a few millimeters away from the olfactory bulb that processes smells. Salinas always suspected that bacteria from the nose could enter the olfactory bulb. After years of curiosity, she decided to confront her suspicions about her favorite model organism: fish.

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

However, these types of samples can be easily contaminated – by bacteria in the laboratory or from other parts of a fish’s body – which is why scientists have had difficulty studying this issue effectively. If they actually found bacterial DNA in the olfactory bulb, they would have to convince themselves and other researchers that it actually came from the brain.

To clarify their basis, Salinas’ team also examined the microbiomes of the fish’s entire body. They took samples of the fish’s remaining brains, intestines and blood; They even drained blood from the brain’s many capillaries to ensure that any bacteria they discovered were in the brain tissue itself.

“We had to repeat (the experiments) many, many times just to be sure,” Salinas said. The project took five years – but right from the start it was clear that the fish’s brains were not sterile.

As Salinas expected, the olfactory bulb harbored some bacteria. But she was shocked to see that the rest of the brain had even more. “I thought there were no bacteria in the other parts of the brain,” she said. “But it turned out my hypothesis was wrong.” The fish brains housed so much that it only took a few minutes to locate bacterial cells under the microscope. As a further step, her team confirmed that the microbes were actively living in the brain; They were neither dormant nor dead.

Olm was impressed by her thorough approach. Salinas and her team “pursued the same question in all these different ways and with all these different methods – and all of them produced convincing data that there are actually live microbes in the salmon’s brain,” he said.

But if so, how did they get there?

Invasion of the fortress

Researchers have long been skeptical that the brain could have a microbiome because all vertebrates, including fish, have a microbiome a blood-brain barrier. These blood vessels and the brain cells surrounding them are so reinforced that they act as gatekeepers, allowing only some molecules in and out of the brain and keeping out invaders, especially larger ones like bacteria. So Salinas naturally wondered how the brains in her study had become colonized.

By comparing microbial DNA from the brain with DNA collected from other organs, her lab found a subset of species that were found nowhere else in the body. Salinas suspected that these species might have colonized the fish’s brains early in their development, before their blood-brain barriers had fully formed. “In the beginning, anything can go in; “It’s anyone’s business,” she said.