The bacterium Chlamydia pneumoniae, best known for causing pneumonia and sinus infections, may remain in the body for many years and be associated with changes observed in people with Alzheimer’s disease. Evidence supporting this link was presented by a group of researchers in a report published in the journal Nature Communications.

Scientists believe that chronic infection may trigger inflammation capable of damaging nerve cells, and they propose new therapeutic approaches — both by treating inflammation and through early intervention with antibiotics, Euronews reports.

The study found that Chlamydia pneumoniae bacteria can appear in the retina — a thin layer of nervous tissue at the back of the eye responsible for processing visual signals. Because the retina is part of the central nervous system and can be examined noninvasively, it offers a rare opportunity to observe brain-related biological processes in real time.

The research team discovered that the presence of bacteria in this tissue is linked to inflammatory immune activity, neuronal cell death, and cognitive decline. Maya Koronyo-Hamaoui, professor of neurosurgery at Cedars-Sinai Health Sciences and the study’s lead author, said: “Repeated monitoring of Chlamydia pneumoniae allowed us to identify a previously unknown connection between bacterial infection, inflammation, and neurodegeneration.”

“The eye is a mirror of the brain, and this study shows that bacterial infection in the retina and chronic inflammation may reflect and predict brain disease, supporting the use of retinal imaging as a noninvasive way to identify individuals at risk of developing Alzheimer’s disease,” she added.

To test this hypothesis, researchers combined advanced imaging techniques with genetic and proteomic analyses and examined retinal tissue from 104 individuals with normal cognition, mild cognitive impairment, and Alzheimer’s disease. The results showed that bacterial levels in the retina and brain of people with Alzheimer’s disease were significantly higher than in individuals with normal cognitive function, and higher bacterial loads were associated with more severe brain changes and greater cognitive decline.

Genetic factors also played a role: higher bacterial levels were more frequently observed in individuals carrying the APOE4 gene, a well-known risk factor for Alzheimer’s disease.

Laboratory experiments further revealed that infection with Chlamydia pneumoniae increased inflammation, promoted neuronal cell death, and worsened cognitive function, suggesting that the bacteria may accelerate disease progression. The infection also stimulated the production of beta-amyloid protein, which accumulates in the brains of people with Alzheimer’s disease.

Timothy Crowther, a research professor at Cedars-Sinai Medical Center and co-author of the study, said the discovery “opens the possibility of targeting the inflammatory axis for Alzheimer’s disease treatment.”

The findings suggest that addressing chronic bacterial infection and inflammation may become a promising therapeutic strategy and confirm the potential of retinal scanning as a noninvasive tool for diagnosing and monitoring the disease.