People with Alzheimer's May Have More Bacteria in Their Brains

brain, neurons
(Image credit: Naeblys/

People with Alzheimer's disease may have higher levels of bacteria in their brains compared to people without the condition, a small new study suggests.

Although more research is needed to confirm the findings, the study may provide evidence to support the hypothesis that inflammation — including inflammation from bacterial infections — contributes to Alzheimer's disease, the study's researchers said.

Alzheimer's is a progressive brain disease in which brain cells become damaged and die, leading to shrinkage of the brain, according to the Mayo Clinic. Exactly what causes this brain cell death is not known, but it's thought that abnormal buildup of a protein called beta-amyloid plays a role. [6 Big Mysteries of Alzheimer's Disease]

Still, some researchers now think that inflammation in the brain may also contribute to Alzheimer's.

"Alzheimer's brains usually contain evidence of neuroinflammation, and researchers increasingly think that this could be a possible driver of the disease, by causing neurons in the brain to degenerate," study co-author David Emery, a researcher from the University of Bristol in the United Kingdom, said in a statement.

Such inflammation could be a reaction to bacteria in the brain, the researchers said. Although the body's blood-brain barrier typically prevents microorganisms and certain chemicals from entering the brain, this barrier may not work perfectly in people at risk for Alzheimer's disease, and bacteria may in fact get into the brain, the researchers said.

In the new study, the researchers analyzed postmortem brain tissue samples from eight people who had Alzheimer's disease and six people who did not have the condition. (All the participants had declared that their brain tissue should be donated to medical research after their death.) The researchers used DNA sequences to detect bacterial genes.

The scientists found that the brains from the Alzheimer's patients had seven times more bacterial genetic sequences than the brains of the people who didn't have the disease.

Much of this difference was due to the Alzheimer's patients having higher levels of bacteria belonging to a family called Actinobacteria. Specifically, the researchers found higher levels of a bacteria called Propionibacterium acnes, which is linked to acne, but has also been found to grow in the brain and may cause inflammation in the body.

The characteristics of P. acnes would make it "a good candidate for a bacterial source of neuroinflammation" in the brains of Alzheimer's patients, the researchers said.

However, the new study does not prove that bacteria play a role in the development of Alzheimer's, the scientists said. More studies are needed to better determine the amount of bacteria present in the brain when a person does and does not have Alzheimer's, and whether these bacteria are involved in brain diseases, the researchers said.

The study was published June 20 in the journal Frontiers in Aging Neuroscience.

Original article on Live Science.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.