Study Reveals Active Microbial Communities in Fish Brains

Researchers at the University of New Mexico in the United States recently reported in the journal Science Advances that active microbial communities have been found in the brains of wild and lab-raised members of the salmon family, including European rainbow trout, Chinook salmon, and Arctic char. In the brains of lab-raised rainbow trout, more than half of the bacteria may come from their blood and intestines, suggesting that microbes from other parts of the body can cross the blood-brain barrier and take up residence in the brain.

Previously, animal brains were thought to be sterile, with any bacterial invasion typically associated with disease. However, increasing evidence suggests that microbes invading the brain may be linked to diseases such as Alzheimer's in humans. This new discovery implies that bacteria in fish brains may not always be bad news. In most cases, despite the presence of microbes in the skulls of these fish, they appear to be healthy.

The researchers suggest that bacteria in the brain may help fish perceive microbial cues in their environment, aiding migratory fish in navigating river systems.

Bacterial testing was conducted on brain samples of rainbow trout, with blood being extracted from the animals first to prevent contamination. Analysis of genetic material from four brain regions indicated that bacterial levels in the rainbow trout brain were similar to those in the spleen but only one-thousandth of the levels found in the gut. Microbial communities were also found in the brains of wild rainbow trout, Atlantic salmon, Chinook salmon, and Arctic char, although these communities differed from those in lab-raised trout and may originate from different organs.