Bacterial differences reported in Alzheimer’s brains

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Researchers from the University of Bristol (Bristol, UK) have investigated bacterial presence in post-mortem brain tissues from Alzheimer’s patients using next-generation sequencing. Their findings report a higher presence of Actinobacteria in Alzheimer’s disease (AD) in comparison with control samples, indicating the possibility that bacterial infections may contribute to AD.

Alzheimer’s is a well-recognized degenerative disease of the brain that has frequently been associated with neuroinflammation. However, the mechanisms leading to the occurrence of inflammation in AD patients have been associated to a wide array of possible risk factors.

David Emery (University of Bristol), an author in the recently published study in Frontiers in Aging Neuroscience, believes that “neuroinflammation in the brain may be a reaction to the presence of bacteria.” In this study, the authors suggest the possibility of Propionibacterium  acnes as a contributing factor for neuroinflammation.

P. acnes are Gram-positive bacterium associated with human skin and mouth microflora. It is believed to have a stimulatory role in the innate immune system by secreting chemotactic and proinflammatory cytokine-inducing factors, which are thought to play a role in inflammatory infiltration.

The researchers compared control patients to AD patients by extracting DNA from frozen tissue samples. Universal bacteria primers (16S rRNA) were then used to assess the bacterial presence in the participant’s brain from the widest taxonomical coverage. This which included major phyla commonly found in the human microbiome: Actinobacteria, Bacteroidetes, Firmicutes, Fusobacteria and Proteobacteria.

The group found a 5–10-fold increase in Actinobacteria population in AD brains compared with controls, whereas Proteobacteria exhibited an inverse relationship – leading to an overall increase in the ratio of Actinobacteria to Proteobacteria in Alzheimer’s subjects. Other taxa were present in the samples but were considered as too limited to analyze.

The researchers cautioned that “P. acnes is a well-documented contaminant of next-generation sequencing techniques” but that “the lack of significant contact with skin makes contamination an unlikely explanation for the P. acnes content of these data.”

This study is the first to demonstrate that 16S next-generation sequencing is able to analyze the presence of bacterial populations in brain tissue samples, particularly those suggested in AD. However, as concluded by co-author Shelly Allen (University of Bristol): “Larger numbers of brain samples are required, and future studies should also investigate if bacteria are involved in other neurodegenerative diseases involving neuroinflammation.”

Sources: Emery DC, Shoemark DK, Batstone TE et al. 16S rRNA next-generation sequencing analysis shows bacteria in Alzheimer’s post-mortem brain. Front. Aging Neurosci. 9, 195 (2017); www.eurekalert.org/pub_releases/2017-07/f-bfi071717.php

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