Comparative analysis of the blood microbiome in Alzheimer’s disease and healthy individuals

Abstract

Human blood has traditionally been considered a sterile environment. However, evidence in support of a bacterial component in the blood of healthy individuals is steadily accruing. Dysbiosis of this blood-microbiome has been linked to various diseases, i.e., diabetes, cirrhosis and liver- and cardiovascular-disease. Alzheimer’s disease (AD) is an important neurodegenerative condition which, causes dementia in the elderly. According to the pathogen hypothesis, microorganisms may play an important role in the pathogenesis of AD. While the intestinal microbiome could be linked to the onset of AD, the relationship between AD and the blood microbiome remains unexplored. In this study, we performed comparative analyses of the blood microbiome in AD and healthy human individuals, placing an emphasis on AD risk factors and the blood microbial environment, via amplicon and metagenomic sequencing of the 16S rRNA gene. Results indicate that bacterial DNA is present in the blood of both healthy individuals and in AD patients. However, the concentration of bacterial DNA does not differ in Alzheimer’s patients, when compared to healthy participants. Whereas AD patients presented a higher alpha-diversity than healthy participants, beta-diversity did not differ between the groups. Certain haematological factors that represent the blood environment appears to influence bacterial taxonomy and diversity in the blood. Conversely, AD associated risk factors, such as the APOE genotype, does not influence bacterial community structure or taxonomy in the blood of AD patients. Future research should aim to explore the link between the human blood microbiome and the immune system, as well as the relationship between bacterial communities in the blood and those found in the intestinal tract, skin and the oral microbiome.