We usually think of bacteria as harmful, causing infections or illnesses. But hold on—recent research is changing that view, especially when it comes to fish. A new study, published on September 18 in Science Advances, has uncovered something quite unexpected: healthy fish, like salmon and trout, have thriving communities of bacteria inside their brains!
It sounds bizarre, doesn’t it? We’re used to hearing that bacteria in the brain means trouble, like an infection or some disease. But in this case, the fish seem perfectly fine despite having microbes living inside their heads.
Bacteria in Fish Brains: The Shocking Truth
Traditionally, we’ve believed that animal brains are supposed to be sterile—free of any bacteria—with any invasion linked to serious health problems. For example, certain brain microbes have been connected to diseases like Alzheimer’s in humans. But this new discovery shakes up that thinking, at least for fish.
A team of researchers, led by Irene Salinas from the University of New Mexico, decided to dive deep into the brains of rainbow trout to see what they could find. And guess what? They found bacteria—and lots of it! The levels of bacteria in the fish brains were similar to what’s found in the spleen, though much lower than in the gut.
Even more interestingly, more than half of the bacteria found in these lab-grown trout likely came from other parts of the fish’s body, like the gut or blood. This suggests that these microbes can cross the brain’s protective barrier and settle in for a while.
Could These Brain Bacteria Help Fish?
Now, the big question is: what are these bacteria doing in fish brains? Are they harmful, or do they serve a purpose?
One theory is that these brain-dwelling bacteria might actually help fish sense their surroundings. Imagine this: if you’re a migratory fish, swimming upstream, being able to pick up on microbial cues in the water might give you a survival advantage. Salinas and her team believe these bacteria might help fish navigate complex environments.
There’s also variation depending on the fish’s environment. Wild fish like Atlantic salmon and Gila trout had more diverse bacterial communities in their brains compared to lab-grown fish, showing how much the environment could affect their brain microbiome.
Is There a Downside to Bacteria in Fish Brains?
Not everything about this discovery is positive, though. Just like an imbalance of gut bacteria can cause health issues, it’s possible that too much brain bacteria could be bad news for fish. For instance, adult Chinook salmon have been found with amyloid-beta build-up, a protein linked to Alzheimer’s in humans, and higher levels of brain bacteria. Could an overgrowth of these microbes lead to problems as fish age? It’s a possibility researchers are still exploring.
Could Other Animals, or Even Humans, Have Brain Bacteria?
This discovery raises a lot of interesting questions beyond fish. Could other vertebrates—maybe even humans—also have brain microbiomes that we don’t know about? It’s a mind-boggling thought. So far, we don’t have an answer, but it’s definitely an area ripe for exploration.
Salinas also points out that fish are incredibly diverse, so it’s possible that other aquatic animals, like deep-sea fish or sharks, might have even more interesting bacterial communities in their brains. Maybe these microbes help them adapt to their unique, often extreme, environments.
What’s Next?
This study is just the beginning. It challenges a lot of what we thought we knew about how microbes interact with the brain. If fish can have healthy bacterial communities in their brains, it makes you wonder what else we’re missing in the animal kingdom—and maybe even in ourselves.
For now, we’re left with an intriguing mystery: could bacteria be playing an unseen role in the brains of not just fish, but many other creatures, too? Only more research will tell, but it’s exciting to think about what else we might uncover as we continue to explore these surprising bacteria-brain connections.