A common respiratory bacterium found in the eye may play a role in Alzheimer's disease progression, according to new research examining the connection between infection and cognitive decline. Scientists discovered that Chlamydia pneumoniae, a bacterium that typically causes respiratory infections, appears in significantly higher amounts in the eye tissue and brain of people with Alzheimer's disease compared to those with normal cognition.

What Is Chlamydia pneumoniae and How Common Is It?

Chlamydia pneumoniae is a respiratory bacterium that spreads through coughing, sneezing, and close contact, much like cold and flu viruses. It can cause sinus infections, bronchitis, or pneumonia, and because it often presents as atypical pneumonia, it frequently goes undiagnosed or untreated. Between 50 to 70 percent of people have been exposed to this bacterium at some point in their lives.

However, exposure to the bacterium does not automatically lead to Alzheimer's disease. Instead, researchers are investigating whether persistent infection and the inflammation it triggers may contribute to Alzheimer's progression in some individuals.

How Did Researchers Connect Eye Bacteria to Brain Disease?

Dr. Maya Koronyo-Hamaoui, a professor of neurosurgery, neurology, and biomedical sciences at Cedars-Sinai Health Sciences University, explained the reasoning behind this investigation. Her team began by asking whether the eye could serve as a noninvasive window into brain health. Since the brain is protected by bone and difficult to access, the retina, which is directly connected to the brain, offers a more accessible way to monitor neurological changes.

"The idea is to use the eye, the back of the eye, this tissue that is connected directly with the brain, as a surrogate, as a window into brain disorders, to diagnose it early, to help with assessing progression and tracking the disease," explained Dr. Koronyo-Hamaoui.

Dr. Maya Koronyo-Hamaoui, Professor of Neurosurgery, Neurology and Biomedical Sciences at Cedars-Sinai Health Sciences University

The team observed structures resembling bacteria within amyloid plaques, the toxic protein accumulations characteristic of Alzheimer's disease. Genome-wide association studies in humans had previously shown that Chlamydia pneumoniae was present in amyloid plaques of Alzheimer's patients, and the bacterium was known to infiltrate and persist in the brain. However, this was the first study to identify the bacterium in the human retina.

What Did the Study Find?

The research examined approximately 104 individuals across three groups: about 37 people who died with normal cognition, 16 who died with mild cognitive impairment due to Alzheimer's disease, and about 51 with Alzheimer's disease dementia at various stages. The findings revealed a striking pattern in bacterial burden and disease severity.

In people with Alzheimer's disease dementia, the amount of Chlamydia pneumoniae in both the retina and brain was markedly elevated, showing a two- to three-fold increase compared to those with normal cognition. Interestingly, some people with normal cognition carried traces of the bacterium, and those with mild cognitive impairment showed some bacterial presence, but the elevation was not statistically significant in these groups.

The association between bacterial burden and disease progression was particularly striking. Patients who died in earlier stages of Alzheimer's disease had lower bacterial levels in the retina and brain, while those who died in advanced stages, with more widespread amyloid and tau pathology, had elevated bacterial levels.

How Does the Bacterium Trigger Brain Inflammation?

Beyond simply being present, the bacterium appeared to trigger a specific inflammatory response. The presence of Chlamydia pneumoniae was associated with activation of the NLRP3 inflammasome, a cellular mechanism that releases inflammatory molecules called cytokines. These cytokines are known to be toxic to nerve cells and are implicated in Alzheimer's disease progression.

Notably, this inflammatory response was elevated even in people with mild cognitive impairment, suggesting that some individuals may be particularly susceptible to persistent infection and the chronic inflammation it causes. This finding points to a potential mechanism by which infection could contribute to cognitive decline over time.

What Should You Know About This Discovery?

• Exposure Does Not Equal Disease: Having been exposed to Chlamydia pneumoniae, which affects 50 to 70 percent of people, does not mean someone will develop Alzheimer's disease. The research suggests that persistent infection in susceptible individuals may be a contributing factor, not a direct cause.
• The Eye as a Diagnostic Window: If confirmed in future studies, retinal imaging could offer a noninvasive way to detect Alzheimer's-related changes early and monitor disease progression without requiring brain biopsies or invasive procedures.
• Inflammation as a Key Mechanism: The study highlights chronic, low-grade inflammation triggered by persistent bacterial infection as a potential pathway in Alzheimer's disease, opening new avenues for understanding and potentially treating the condition.
• Multiple Factors at Play: Alzheimer's disease is multifactorial, meaning many elements contribute to its development. This bacterium may be one piece of a larger puzzle involving genetics, lifestyle, and other infections.

What Happens Next in This Research?

While the findings are compelling, researchers emphasize that more work is needed to understand the relationship between Chlamydia pneumoniae and Alzheimer's disease. The current study establishes an association, but does not prove that the bacterium directly causes cognitive decline. Future research will need to determine whether treating or preventing persistent infection with this bacterium could reduce Alzheimer's risk or slow disease progression in vulnerable populations.

The discovery also raises questions about other potential infectious agents. The research team noted that other bacteria or viruses could potentially trigger similar inflammatory responses and contribute to Alzheimer's pathology, suggesting that infection may be one of several infectious mechanisms involved in neurodegeneration.