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For decades, scientists have believed the human brain is a sterile fortress, protected from microbial invaders by the robust blood-brain barrier. But a new study, published in Science Advances, challenges this assumption by showing bacteria can’t just make their way to the brain, they can thrive there.

Researchers from the University of New Mexico, led by biologist Irene Salinas, made this startling revelation when studying salmon and trout. Using DNA extraction and microscopic imaging, they identified living bacteria in the fishes’ olfactory bulbs and other brain regions. The results showed the olfactory bulb, which is directly connected to the nasal cavity, harboured bacteria as did deeper-lying brain tissue.

The presence of bacteria in fish brains raised several questions. The foremost was about how they managed to cross the blood-brain barrier. Salinas & co. discovered that many of these microbes possessed unique adaptations that helped them breach the barrier. Some produced molecules called polyamines that can open tight junctions in the barrier fluid; others were able to evade immune responses or outcompete their rivals, ensuring their survival in the brain’s delicate environs.

The group also explored the origins of these brain-dwelling microbes. Some bacteria seemed to have colonised the brain much before the blood-brain barrier had evolved to its present form. Others likely travelled up from the gut or the bloodstream, continuously infiltrating the brain throughout the fishes’ lives. The researchers said the presence of more than one pathway suggests the brain’s microbial community is dynamic, shaped by both early colonisation and ongoing interaction with other bodily systems.

A particularly striking finding was the image of a bacterium caught mid-transit across the barrier, offering direct visual evidence. Some researchers have hypothesised these microbes might be engulfed by immune cells while others have suggested they could play active roles in physiological processes — just like the human gut microbiome does in regulating digestion, immunity, and mood.

Fish are very different from humans yet the study also opens the door to rethinking the brain’s microbiome in vertebrates, including humans. If bacteria can thrive on fish brains, it’s possible they may do so on human brains as well.

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