Bacteria in the brain: do they have a role in the pathogenesis of Alzheimer's disease?

Purpose of review: Worldwide efforts continue to unravel the complex pathological pathways that lead to Alzheimer's disease. The gut-brain-microbiome axis, a communication pathway between the gut, brain and microorganisms, is emerging as a potential mechanism involved in Alzheimer's disease pathogenesis. While the gut microbiome's role in Alzheimer's disease has gained significant attention, the brain microbiome remains relatively unexplored. This review summarizes the latest research on the brain microbiome in Alzheimer's disease.

Recent findings: In the past 4 years, four out of five studies have found bacteria, such as Streptococcus pneumoniae, in postmortem samples of both control and Alzheimer's disease brains, supporting the idea that the brain is not a sterile environment. Two studies report the overabundance of several bacterial phyla, including Proteobacteria and Actinomycetes, in postmortem Alzheimer's disease brains versus controls. One study reported the presence of Borrelia burgdorferi in a subset of Alzheimer's disease cases compared to controls.

Summary: Limitations and challenges persist in studying the brain microbiome, including the lack of standardized assays and data analysis methods, small sample sizes, and inconsistent use of controls for environmental microbial contamination during sample processing. Well designed studies that employ reproducible and rigorous methods are required to elucidate whether microbes are involved in the pathogenesis of Alzheimer's disease.