A bacterium best known for causing pneumonia and sinus infections appears to have a hidden agenda in the brain. Researchers at Cedars‑Sinai discovered that Chlamydia pneumoniae can survive for years inside the retina and cerebral tissue, potentially aggravating the damage linked to Alzheimer’s disease. Their findings, appearing in Nature Communications, hint that tackling chronic infection and the resulting inflammation might open new therapeutic avenues, such as early‑stage antibiotics and anti‑inflammatory strategies.
For the first time, scientists have shown that Chlamydia pneumoniae is capable of migrating to the retina—the light‑sensing layer at the back of the eye. Once there, it sparks immune reactions that promote inflammation, loss of retinal neurons, and a decline in cognitive performance.
Higher Bacterial Load Correlates With Worse Cognition
The team examined retinal samples from 104 individuals, employing high‑resolution imaging, genetic profiling, and protein analyses. Participants spanned the spectrum from normal cognition to mild cognitive impairment and full‑blown Alzheimer’s.
Those diagnosed with Alzheimer’s harbored markedly more Chlamydia pneumoniae in both their retinas and brains compared to cognitively healthy subjects. Moreover, a greater bacterial burden was linked to more extensive brain damage and steeper declines in memory and reasoning.
The spike in bacterial presence was especially pronounced among carriers of the APOE4 allele, a genetic variant that heightens Alzheimer’s risk.
Infection May Speed Up Alzheimer’s Pathology
To probe causality, researchers exposed cultured human neurons and Alzheimer‑model mice to the bacterium. In both systems, infection amplified inflammatory signals, accelerated neuronal death, and worsened behavioral deficits. Notably, the bacteria also boosted production of amyloid‑beta, the sticky protein that forms plaques in Alzheimer’s brains.
Overall, the study suggests that eradicating long‑standing bacterial infections and dampening their inflammatory fallout could represent a fresh treatment strategy. It also strengthens the case for using retinal imaging as a non‑invasive window into Alzheimer’s progression.
Funding Acknowledgment: This work was supported by NIH/NIA grants R01AG056478, R01AG055865, AG056478‑04S1, R01AG075998, an Alzheimer’s Association grant (AARG‑NTF‑21‑846586), the Goldrich and Snyder Foundations, and the Ray Charles Foundation.