Brains are usually thought of as a bacteria-free zone. But that’s not the case for some fish.

Wild and lab-grown members of the salmon family harbor active microbial communities in their brains. More than half of the bacteria found in brains of lab-reared rainbow trout appear to have initially come from their blood and guts. That suggests that microbes from other parts of the body can cross the blood-brain barrier. That layer of cells lies between brain cells and blood vessels. It lets oxygen and nutrients in. Typically, it also keeps bacteria out.

Microbial infection of animal brains has usually been linked to disease. For instance, brain-infiltrating microbes may be linked with ailments such as Alzheimer’s disease in people. But brain bacteria aren’t necessarily bad news for fish. These animals seem mostly healthy despite hosting germs inside their skulls.

The bacteria might even help the fish, says Irene Salinas. She’s an immunologist who studies fish. She works at the University of New Mexico in Albuquerque. Bacteria could help fish pick up on microbial cues in the environment, she says. That might help migratory fish sense molecules that help them navigate rivers.

Bacteria on the brain

Salinas’ team scouted for bacteria in brain samples from rainbow trout.

First, they removed the blood from the animals to avoid contamination. Then, they took tissue samples and tried to grow bacteria from them. The four brain regions they sampled had similar levels of bacteria as found in the spleen. That organ can filter substances, including bacteria, from the blood. The brain had one-thousandth the level of microbes found in fish guts.

Several wild fish — types of salmon and trout — also have bacteria in their brains. But they had different types and amounts than those found in lab-grown trout. These might have come from different internal organs. The researchers shared their findings September 18 in Science Advances.

The team grew a total of 54 types of bacteria from lab-reared fish.

That suggests that fish brains have a microbiome, or microbial community. Genetic analyses also revealed signs that bacteria adapt to living in the brain.

The DNA of these microbes hinted at structures that may help them cross the blood-brain barrier. Whether the germs settle in and reproduce or just constantly trickle in from other organs is not yet known.   

But brain bacteria may not always be beneficial for fish. The brains of Chinook salmon nearing death tend to have more bacteria than those of juveniles. These aging brains also can have a buildup of amyloid-beta. It’s a protein involved in Alzheimer’s disease.

Sometimes gut microbes go out of whack. It’s possible something similar happens in the brain, leading to problems, Salinas says. 

The new findings open up questions about whether only fish have bacteria in the brain. Perhaps other animals have brains chock full of microbes, too. And when it comes to fish, “there’s a lot of diversity of different fishes on the planet,” Salinas says. Maybe deep-sea fish or sharks have unique brain bacteria that help them live far below the surface.