Microbes on the Mind: How the Brain Microbiome Affects Dementia Risk

Everyone is aware that we have a microbiome within our digestive tract. It’s a whole ecological system unto itself containing bacteria, viruses and fungi. For decades, medical students learned that the lungs, the brain and other organ systems did NOT harbor such micro-organisms. But now, evidence is mounting that was a misconception. There is clearly a lung microbiome. Scientists are beginning to find evidence of a brain microbiome as well. In addition, new research points to the power of the gut microbiome in altering brain health.

Microbiome Changes Are Linked to Parkinson Disease Risk

Doctors think of Parkinson disease as a neurological condition, but new research on the gut microbiome may allow them to diagnose it much earlier than they currently do. That’s because scientists have identified predictable changes in the gut microbiome of patients even before they have neurological symptoms (Nature Medicine, March 31, 2026).

Parkinson disease has become more common over the past few decades and now affects more than 8.5 million people around the world. Brain damage, which starts in the substantia nigra, leads to problems with balance, movement, and even memory.

Scientists at University College London wondered why only one-fifth of people with a genetic predisposition to Parkinson disease actually develop symptoms. They compared the microbiomes of 271 patients with Parkinson disease, 43 people with that gene (dubbed GBA1) and 150 healthy people serving as a control group. There were significant differences in the abundance of various gut microbes between patients and healthy people. Moreover, more than a quarter of the gut microbes identified had  an intermediate abundance among individuals at risk for Parkinson disease. The researchers corroborated these changes in the microbiome among independent cohorts of patients and healthy people from the US, Turkey and Korea.

Effecting changes in the microbiome is theoretically possible, although it appears to be challenging. The abundance of gut microbes does change based on diet, but people seem to have some difficulty making those changes permanent. Perhaps researchers will figure out a diet that can help avert Parkinson disease. In the meantime, we can contemplate the implications of a brain microbiome. That is even more difficult to study.

Fish Have a Brain Microbiome

We can imagine that brain infections would wreak havoc with the neurons. But even healthy brains aren’t devoid of microbes. A researcher in New Mexico has painstakingly confirmed the presence of a microbiome in the brains of fish. According to Dr. Irene Salinas, the mucous membranes of the nose, which encounter a lot of microbes in the course of a day, are located extremely close to the olfactory bulb of the brain. In the salmon or trout, there may be no more than a few millimeters of separation. The olfactory bulbs are not the only areas of the brain to host microbes, though. Salinas and her laboratory found living microbes in many regions of the brains of their fish. She hypothesizes that if fish have a brain microbiome, then humans might also. We’ll need more research in the future to determine if that is the case.

Bad Bugs Cause Brain Problems

Whether or not the human brain normally hosts a microbiome, when the brain succumbs to infection the results can be devastating. Investigators have long suspected a link between the virus that causes cold sores and the risk for Alzheimer disease. A research paper published in the journal Alzheimer’s & Dementia describes a number of cases linking microbial infection of the brain to cognitive decline and dementia (Alzheimer’s & Dementia, Nov. 2023). The microbes included Borrelia burgdorferi which causes Lyme disease and Cryptococcus neoformans, a fungal infection. One of the cardinal signs of Alzheimer disease is the accumulation of amyloid plaque in the brain. The scientists point out that Abeta peptide, a building block for amyloid plaque, is produced as part of the brain’s immune response to infection. It can trap pathogens and protect host cells. So the much-feared amyloid build-up may be a sign of overwhelming infection and resulting inflammation rather than the prime mover of dementia.

The researchers note,

“the combined weight of evidence argues that the brain houses its own microbiome, and that infection may contribute to the neuroinflammation and neurodegeneration seen in AD.”

They plan ambitious further studies to determine exactly how the brain microbiome may be involved in the development of dementia.

Learn More

To learn more about Parkinson disease, you may want to listen to our interview with Dr. Ray Dorsey. It is Show 1259: Preventing and Treating Parkinson’s Disease. There is more information about the microbiome in Show 1302: Managing the Microbiome for Better Health.