Study links eye bacteria to higher risk of Alzheimer’s disease

A new scientific study has identified a potential link between the presence of a common bacterium in the retina and an increased risk of cognitive decline and Alzheimer’s disease, strengthening the theory that the eye could serve as an early window into brain health.

The research, led by scientists at Cedars-Sinai Medical Center in the United States, detected the bacterium Chlamydia pneumoniae –typically associated with respiratory infections — inside the nerve tissue at the back of the eye responsible for vision. The bacterium was found at higher levels in individuals diagnosed with Alzheimer’s disease.

Previous studies had identified the same bacterium in the brains of Alzheimer’s patients. However, its discovery in the retina is particularly significant because the retina is considered a direct extension of the central nervous system.

Neuroscientist Maya Coronio-Hamowe, a co-author of the study, said the findings support the idea that changes in the eye may reflect processes occurring in the brain. She noted that a chronic bacterial infection in the retina could signal pathological changes linked to Alzheimer’s disease.

The research team analyzed eye and brain tissue samples from 104 individuals after death. Participants included people with Alzheimer’s disease, others with mild cognitive impairment, and individuals with no cognitive issues.

The results showed a clear association between higher bacterial levels in both eye and brain tissues and the severity of cognitive decline. The study also found that individuals carrying genetic mutations associated with a higher risk of Alzheimer’s had increased levels of the bacterium.

Laboratory experiments and animal studies further suggested how the bacterium might contribute to disease processes. Infection with Chlamydia pneumoniae was linked to increased neuroinflammation, accelerated loss of nerve cells, and elevated levels of beta-amyloid protein, which is known to form plaques in the brains of Alzheimer’s patients.

Researchers emphasized that the bacterium may not be a direct cause of the disease but could act as a contributing factor that accelerates inflammatory and degenerative changes in the brain.

Although the study did not directly test new diagnostic methods, the findings open the door to the possibility of developing non-invasive screening tools based on retinal examinations. Such approaches could one day help detect early warning signs of dementia and Alzheimer’s disease through the eye.