Scientists at Cedars-Sinai have found traces of a common eye bacterium in people with Alzheimer's disease, and the discovery is shifting how researchers think about what causes cognitive decline in aging.
The bacterium, Chlamydia pneumoniae, normally causes respiratory infections. But researchers found higher levels of it in the retinal tissue of Alzheimer's patients, and in lab studies, the infection triggered the hallmark damage associated with the disease: inflammation, nerve cell death, and buildup of amyloid-beta plaques—the toxic proteins that accumulate in Alzheimer's brains.
What makes this finding particularly promising isn't just the link itself, but where it was found. The eye's retina sits behind a barrier that mirrors the brain's own protective wall, making it accessible in ways the brain isn't. "The eye is a surrogate for the brain," explains Maya Koronyo-Hamaoui, the study's lead author. This means doctors might eventually use retinal imaging—a simple, noninvasive scan—to identify who's at risk for cognitive decline before symptoms appear.
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Start Your News DetoxThe research, published in Nature Communications, adds weight to a growing recognition that chronic infections may play a significant role in neurodegeneration. This isn't the first bacterium implicated: gum disease bacteria has been detected in Alzheimer's brains, and herpes simplex virus has been linked to increased disease risk. But each new connection fills in more of a picture that was largely invisible a decade ago.
There's an important caveat. The study shows correlation, not causation—it's not yet clear whether the bacterium directly triggers disease or simply flourishes in a brain already compromised by age and inflammation. The team did find the bacterium was more common in people carrying the APOE4 gene variant, a well-established Alzheimer's risk factor, which suggests some interaction between genetics and infection.
If the infection does play a direct role, the implications are significant. "This discovery raises the possibility of targeting the infection-inflammation axis to treat Alzheimer's," says Timothy Crother, co-author of the study. That could mean antibiotics, anti-inflammatory drugs, or vaccines—interventions that are far simpler than trying to clear amyloid plaques after they've accumulated.
The next steps are clear: researchers need to determine whether treating the infection actually slows or prevents cognitive decline. But for the first time, there's a testable mechanism and a noninvasive way to spot it.
