Common Cold Bacteria Found in Eyes May Worsen Alzheimer's Disease

A study has found that bacteria causing the common cold can remain in the eyes and worsen Alzheimer's disease.

According to the Daily Mail on Monday (local time), researchers at Cedars-Sinai Medical Center in Los Angeles analyzed postmortem eye tissue donated by Alzheimer's patients, mild cognitive impairment patients, and healthy individuals to reach these findings.

The bacteria in question is Chlamydia pneumoniae, so common that 80 percent of adults will be infected at least once in their lifetime. It typically causes only mild symptoms such as sore throat, fatigue, and runny nose before passing.

When researchers analyzed tissue from 95 postmortem donors, they found this bacteria was detected 2.9 to 4.1 times more frequently in the retinas and brains of Alzheimer's patients compared to healthy individuals. The retina is the light-sensing tissue at the back of the eye that shares developmental origins with the brain, making it essentially an extension of the brain.

Higher bacterial levels correlated with more severe brain damage and cognitive decline. Previous studies had also found this bacteria in the brain tissue of Alzheimer's patients, particularly near amyloid plaques that cause memory loss and confusion.

Unlike other bacteria, Chlamydia pneumoniae can live inside human cells. It may evade the immune system and hide in the body for extended periods, potentially causing harmful inflammation.

Researchers infected human nerve cells with this bacteria in laboratory experiments. The results showed activation of potent inflammatory pathways associated with Alzheimer's and increased levels of disease-related proteins. The team also used computer analysis to determine whether specific patterns in the retina could aid in Alzheimer's diagnosis.

"We consistently found Chlamydia pneumoniae in human tissue, cell cultures, and animal models," said Maya Koronyo-Hamaoui, professor at Cedars-Sinai Health Sciences. "We discovered a new link connecting bacterial infection to inflammation and neurodegeneration."

"The eye is a window into the brain," she added. "By observing bacterial infection and chronic inflammation in the retina, we can understand the brain's condition and predict how far the disease has progressed."

Co-author Professor Timothy Bhattacharya said, "This discovery opens a path to treating Alzheimer's by preventing infection and inflammation."