The human gut microbiota plays an important role in the body, communicating with the brain and maintaining the immune system through gut-brain axis. So it's not far-fetched to suggest that bacteria may play an even bigger role in our neurobiology.
Fishing for bacteria
For many years, Irene Salinas was fascinated by a simple physiological fact: The distance between the nose and the brain is quite small. The evolutionary immunologist, working at the University of New Mexico, studies the mucosal immune system in fish to better understand how human versions of these systems, such as the mucosa, work. our intestinal lining and nasal cavity. The nose, she knew, was full of bacteria and they were “very, very close” to the brain — just millimeters away from the olfactory bulb, which processes smell. Salinas always had a hunch that bacteria could leak from the nose into the olfactory bulb. After years of curiosity, she decided to face her doubts about her favorite model organism: fish.
Salinas and her team started by extracting DNA from the olfactory bulbs of salmon and trout, some caught in the wild and some raised in her lab. (Key contributions to the study were made by Amir Mani, the paper's lead author.) They plan to look up DNA sequences in databases 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. That is why scientists must make efforts to research this topic 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 microbiome. They sampled the remains of the fish's brain, intestines and blood; They even siphoned blood from many of the brain's capillaries to ensure that any bacteria they detected resided in the brain tissue itself.
“We had to go back and redo (the experiments) many times to make sure,” Salinas said. The project lasted five years, but even in the early days, it was clear that fish brains were far from barren.
As Salinas predicted, the olfactory bulb contained some bacteria. But she was shocked to see that the rest of the brain had even more. “I think other parts of the brain will be bacteria-free,” she said. “But it turns out my hypothesis was wrong.” Fish brains contain so much that it only takes a few minutes to locate bacterial cells under a microscope. As an additional step, her team confirmed that the bacteria were actively living in the brain; they are not in a state of sleep or death.
Olm was impressed with their thorough approach. Salinas and her team turned to “the same question, from all different ways, using all different methods — all of which produced convincing data that there are indeed bacteria living in salmon brain,” he said.
But if so, how did they get there?
Invade the fort
Researchers have long been skeptical that the brain might have a microbiome because all vertebrates, including fish, have blood-brain barrier. These blood vessels and surrounding brain cells are reinforced to act as gatekeepers allowing only certain molecules into and out of the brain and keeping out invaders, especially larger molecules. like bacteria. So Salinas naturally wondered how the brain in her study had been invaded.
By comparing microbial DNA from the brain with DNA collected from other organs, her lab found a subset of species that appeared nowhere else in the body. Salinas hypothesizes that these species may have colonized fish brains very early in development, before their blood-brain barrier is fully formed. “From the beginning, anything can get in; it is a free for all,” she said.