The human gut microbiome plays a critical role in the body, interacting with the brain and maintaining the immune system through the gut-brain axis. So it’s not totally far-fetched to suggest that microbes can play an even bigger role in our neurobiology. Fishing for Microbes For years, Irene Salinas has been 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 in fish to better understand how the human version of the system, such as the lining of the gut and nasal cavity, works. The nose, he found out, is full of bacteria, and is “very close” to the brain – just millimeters from the olfactory bulb, which processes smell. Salinas must have had a hunch that bacteria could leak from the nose into the olfactory bulb. After years of curiosity, he decided to face his suspicions in his favorite model organism: 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 the laboratory. (An important contribution to the research was made by Amir Mani, the lead author of the paper.) They plan to search the database for DNA sequences to identify any microbial species. in the laboratory or from other parts of the fish’s body – that’s why scientists struggle to study it effectively. If he found bacterial DNA in the olfactory bulb, he would have to convince himself and other researchers that it came from the brain. They sampled other fish brains, guts, and blood; they even drew blood from many of the brain’s capillaries to ensure that any bacteria they found resided in the brain’s own tissue. “We have to go back and do it again [the experiments] Several times just to be sure,” Salinas said. The project took five years—but early on it was clear that the fish’s brain was not sterile. As Salinas expected, the olfactory bulb hosts some bacteria. But he was surprised to see other brains “I thought the other part of the brain wouldn’t have bacteria,” he said sure The microbes are active in the brain; they’re not dormant or dead. Olm is impressed by the thorough approach. believe that there are microbes living in the brain of salmon,” he said. did they get there? Invading the FortressResearchers have long been skeptical that the brain could have a microbiome because all vertebrates, including fish, have a blood-brain barrier. The blood vessels and surrounding brain cells are strengthened to act as gatekeepers that allow only certain molecules in and out of the brain and prevent invaders, especially larger ones like bacteria, from getting out. So Salinas must have wondered how the brains in his study had been colonized. By comparing microbial DNA from the brain with that collected from other organs, his lab discovered a subset of species not seen elsewhere in the body. Salinas hypothesizes that the species may colonize the fish’s brain early in its development, before the blood-brain barrier is complete. “Initially, anything can come in; it’s free for all,” he said.
