Common bacteria discovered in the eye linked to cognitive decline

Chlamydia pneumoniae—a common bacterium that causes pneumonia and sinus infections—can linger in the eye and brain for years and may aggravate Alzheimer’s disease, according to a study from Cedars-Sinai. Published in Nature Communications, the discovery suggests this bacterium can amplify Alzheimer’s disease and points to potential interventions including inflammation-limiting therapies and early antibiotic treatment.

Bacteria reach the eye and brain

The study shows for the first time that Chlamydia pneumoniae can reach the retina—the tissue lining the back of the eye—where it triggers immune responses linked to inflammation, nerve cell death and cognitive decline.

“Seeing Chlamydia pneumoniae consistently across human tissues, cell cultures and animal models allowed us to identify a previously unrecognized link between bacterial infection, inflammation and neurodegeneration,” said Maya Koronyo-Hamaoui, Ph.D., professor of Neurosurgery, Neurology, and Biomedical Sciences at Cedars-Sinai Health Sciences University and the leading, senior author of the study.

“The eye is a surrogate for the brain, and this study shows that retinal bacterial infection and chronic inflammation can reflect brain pathology and predict disease status, supporting retinal imaging as a noninvasive way to identify people at risk for Alzheimer’s.”

How the study was conducted

To conduct the study, researchers used advanced imaging, genetic testing and protein analysis to examine retinal tissue from 104 individuals, some with normal cognition, some with mild cognitive impairment and some with Alzheimer’s disease.

They found significantly higher levels of Chlamydia pneumoniae in the retinas and brains of people with Alzheimer’s disease than they found in people with normal cognition. The higher the bacterial levels detected, the more severe the brain changes and cognitive decline investigators found.

Higher levels of the bacterium were more common in people who carried the APOE4 gene variant, a known risk factor for Alzheimer’s disease.

Links to inflammation and treatment ideas

Investigators also studied human neurons in the lab and in laboratory mice with Alzheimer’s disease. In both, infection with Chlamydia pneumoniae increased inflammation, nerve cell death and cognitive decline, showing the bacterium can accelerate disease processes. The infection also triggered production of amyloid-beta, the protein that accumulates in the brains of people with Alzheimer’s.

The findings were driven by co-first authors Bhakta Gaire, Ph.D., and Yosef Koronyo, MSc. “This discovery raises the possibility of targeting the infection-inflammation axis to treat Alzheimer’s,” said Timothy Crother, Ph.D., co-corresponding author of the study and research professor at Cedars-Sinai Guerin Children’s and the Department of Biomedical Sciences at Cedars-Sinai.

The findings suggest that targeting chronic bacterial infection—and the inflammation it triggers—could represent a new treatment strategy. The research also supports potential use of the retina as a noninvasive way to help diagnose and monitor the disease.