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Researchers at the University of New Mexico have identified the presence of bacteria in the healthy brains of fish. Understanding this connection between bacteria and animal brains could have future implications for the study of Alzheimer's disease.

UNM Professor Irene Salinas has studied mucosal immune systems in vertebrate animals for a long time. Her laboratory, which mostly works with rainbow trout, investigates the interactions between the immune and nervous systems, a field called neuroimmunology. She focuses on host-microbiota interactions, in other words, how microbes that live in symbiosis with animals, modulate nervous and immune functions.

For a few years, Salinas' team had looked closely at neuroimmune interactions in the olfactory-brain axis. In 2019, Ph.D. student Amir Mani joined Salinas' team. Soon, they started wondering about bacterial impacts on the brain and whether bacteria in the nose might make their way into the brain. Using previous research as their guide, they quickly realized that healthy fish have bacteria in their blood and other internal organs.

The idea of internal microbiomes is controversial because in humans, bacteria are only thought to move into our bodies when we are sick. However, the team thought fish may be different in that they may allow bacteria to move into their bodies without causing disease. That spurred the hypothesis that fish may also have bacteria in their brains, even when they are perfectly healthy.

"Our original hypothesis was that we may find some bacteria in the part of the brain called the olfactory bulb, which is the one part of the brain connected to the nose," Salinas said. "However, we didn't expect to see bacteria in other parts of the brain. In fact, the olfactory bulb was the region where bacterial loads were lowest within the brain.

Their study, "A brain microbiome in salmonids at homeostasis," recently in Science Advances, describes in detail the living bacterial community in the brain of healthy salmonids.

Brain bacteria in healthy salmonids are present at a density similar to that of the spleen and, as expected, much lower in the gut. So, where do the brain bacterial communities originate?

Through computational analyses, the team determined that more than 50% of the diversity in the brain can be attributed to gut and blood bacterial communities. Since the blood circulates through the brain constantly, it is not surprising that bacteria use blood as a carrier to reach the fish brain.

In the study, the authors were able to visualize bacteria using fluorescence microscopy. Interestingly, some of them were observed crossing the blood brain barrier. This supported the predictions that the blood is likely a very important conduit for bacteria to reach the brain. It also suggested the brain bacteria may regularly be replenished from blood sources at a steady rate.

While the initial experiments detected bacterial DNA in the trout brain, it was critical to show that these bacteria are alive. Using culturomics, which aims to test as many growth conditions as possible, Mani obtained more than 50 identical bacterial isolates from healthy trout brains as part of the study.

This bacterial biobank, in a freezer in Salinas' lab, represents an invaluable resource for any researcher wanting to look further into the functions of these isolates.

"We put a lot of effort into optimizing this to work for the blood and low microbial tissues to investigate this discovery," said Mani. "We spent almost two years optimizing the methods used in the study. We hope this work, therefore, helps the entire field in future investigations based on our groundwork."

To support their findings from laboratory rainbow trout, Salinas' team sought to sample several other salmonid species from many different locations around the world, including Gila Trout from New Mexico, Chinook salmon from Oregon in its natural habitat, Atlantic salmon from Norway and rainbow trout from the Czech Republic. In all cases, bacteria were found in the brains of these healthy fish, substantiating the team's discovery.

Despite the encouraging findings in salmonids, researchers are still uncertain whether brain microbiomes occur at homeostasis in other fish, other vertebrates or in humans.

"Many others have tried to make the claim that diseased human brains have bacteria," said Mani. "And that has been a real controversy in the Alzheimer's disease field because there have been a lot of papers showing that brain autopsies from Alzheimer's disease patients have certain suites of bacteria in their brain."

However, studies using human brain autopsies are very hard to control, and it is therefore impossible to rule out contaminations during sample collection or due to post-mortem changes. The researchers took all precautions to make sure that the bacteria they were recovering from the fish brains were actually in the brain and not a contamination artifact.

This study is impactful because it shows that bacteria are naturally present in the brains of fish without causing harm. This finding might help scientists better understand how bacteria interact with the brain in other animals, including humans. Researchers think this could offer insights into diseases like Alzheimer's, where previous studies have found bacteria in the brains of affected people.

"I think there are a lot of applications for human health, but there are also a lot of really interesting questions that we can ask about the importance of this brain microbiota in fundamental fish behavior and physiology," added Salinas.

The team is currently trying to seek funding to support this line of work further and is also excited to collaborate with any other researchers to continue answering questions about brain microbiomes.