The common bacterium Chlamydia pneumoniae, which usually causes pneumonia and sinus infections, may be linked to cognitive decline and Alzheimer’s disease when it is present in the retina of the eye. This conclusion was reached by scientists from the Cedars-Sinai Medical Center in the United States. The study was published in the journal Nature Communications (NatCom).

Previously, Chlamydia pneumoniae had already been found in the brains of people with Alzheimer’s disease. Now, for the first time, researchers have shown that the bacterium is also detected in increased amounts in the retina — the tissue at the back of the eye responsible for processing visual signals. Its concentration was higher in people with a confirmed Alzheimer’s diagnosis and correlated with the severity of cognitive impairment.

“The eye can be considered a ‘mirror’ of the brain. Our study shows that bacterial infection of the retina and chronic inflammation may reflect pathological processes in the brain and predict disease status,” said neuroscientist Maya Koronyo-Hamaoui from Cedars-Sinai.

In the first stage, scientists analyzed post-mortem samples of eye and brain tissue from 104 individuals. These included patients with Alzheimer’s disease, people with mild cognitive impairment, and individuals without signs of dementia. The presence of C. pneumoniae in the retina and brain was clearly associated with an Alzheimer’s diagnosis, while in people with mild impairment the link was weaker.

The researchers also found that carriers of a variant of the APOE gene, which increases the risk of Alzheimer’s disease, had higher levels of the bacterium in their tissues. This points to a possible interaction between genetic predisposition and infectious factors.

The team then conducted experiments on neuron cultures and animal models. They showed that infection with Chlamydia pneumoniae intensified inflammation, accelerated nerve cell death, and worsened cognitive function. In addition, the infection was associated with increased levels of beta-amyloid — the protein that forms the toxic plaques characteristic of Alzheimer’s disease in the brain.

The authors emphasized that the discovery points to a potentially new direction for diagnosis and therapy. If the link between infection, inflammation, and neurodegeneration is confirmed, the retina could become a convenient non-invasive “window” for detecting dementia risk at early stages.

The scientists plan to continue their research to clarify the mechanisms of the bacterium’s effects and to assess whether combating chronic infections could slow the development of neurodegenerative diseases.