Heena Cho | April 7th, 2025

Researchers at the University of New Mexico have made a surprising discovery: salmon and trout brains house bacteria. This challenges the long-held belief that the formation of a microbiome in the vertebrate brain is impossible due to an impenetrable protective layer called the blood-brain barrier. These bacteria aren’t just present — they’re alive and active. In fact, researchers even captured an image of a bacterium crossing the blood-brain barrier under a microscope!

The study revealed that these microbes have developed special mechanisms to sneak past the blood-brain barrier, such as producing molecules that open tiny junctions in the barrier. But how did they get there in the first place? One possibility is early invasion: they may have colonized the fish brain early on in life, when the blood-brain barrier was still forming. Another possibility is that the bacteria leaked in from the gut or blood during later development.

Fish aren’t humans, but this intriguing discovery raises the possibility that bacteria might also live in human brains. Human gut microbes influence brain activity via the gut-brain axis. Could bacteria reside in human brains as well? If so, what role might they play? These are fascinating questions for future research in evolutionary biology.

References

‌Wu, D., Chen, Q., Chen, X., Han, F., Chen, Z., & Wang, Y. (2023). The blood–brain barrier: structure, regulation, and drug delivery. Signal Transduction and Targeted Therapy, 8(1), 1–27. https://doi.org/10.1038/s41392-023-01481-w 

‌Yasemin Saplakoglu. (2023, November 21). In the Gut’s “Second Brain,” Key Agents of Health Emerge. Quanta Magazine. https://www.quantamagazine.org/in-the-guts-second-brain-key-agents-of-health-emerge-20231121/

Yasemin Saplakoglu. (2024, December 2). Fish Have a Brain Microbiome. Could Humans Have One Too? Quanta Magazine. https://www.quantamagazine.org/fish-have-a-brain-microbiome-could-humans-have-one-too-20241202/