In recent years, researchers have become a great deal more interested in analysis of the various microbiomes that populate the human body. The gut microbiome is clearly influential on long-term health, and changes in detrimental ways with age. There are many other niches of the body in which microbes dwell, however. Here researchers take a look at the microbes that can be found in the bloodstream of healthy individuals. This also can be seen to change with age, and we might suspect that these changes are harmful in some way. The challenge lies in demonstrating that to be the case, of course.
Metagenomic approaches for studying microbial genomes are being used to determine the potential roles of the gut microbiome, skin, and blood in chronic inflammatory diseases. According to the inflammatory theory, inflammation underlies many chronic diseases, which means that lipopolysaccharides (LPS) from inflammatory cytokines and bacteria are present in the blood. This gives rise to the probability that bacteria act as inflammatory sources and might be present in the blood even in a healthy state. Evidence of a dormant blood microbiota comes from its direct assessment using culture-independent methods, including the detection of blood (or tissue) microbial macromolecules such as the 16S ribosomal RNA (rRNA) gene and direct visualization of cells using ultramicroscopic methods. Since human blood has traditionally been thought to be a completely sterile environment composed only of blood cells, platelets, and plasma, the presence of microbes in the blood has consistently been interpreted as an indication of infection. Although it is a controversial concept, there is increasing evidence for the existence of a healthy human blood microbiota.
Although evidence indicating the presence of a microbial component in the blood of healthy human individuals is steadily accumulating, the influence of age on healthy human blood microbiota composition remains ambiguous. Aging affects both the host and microbiome physiologically, and host-microbiome interactions may affect aging. Most contemporary research has focused on age-related microbiomes in the human gut. The aim of this study was to demonstrate the presence of a blood microbiota in healthy individuals and to identify bacteria at the phylum and class levels using next generation sequencing data.
Using 37 samples from 5 families, we extracted sequences that were not mapped to the human reference genome and mapped them to the bacterial reference genome for characterization. Proteobacteria account for more than 95% of the blood microbiota. The results of clustering by means of principal component analysis showed similar patterns for each age group. We observed that the class Gammaproteobacteria was significantly higher in the elderly group (over 60 years old), whereas the relative abundance of the classes Alphaproteobacteria, Deltaproteobacteria, and Clostridia was significantly lower. In addition, the diversity among the groups showed a significant difference in the elderly group. This result provides meaningful evidence of a consistent phenomenon that chronic diseases associated with aging are accompanied by metabolic endotoxemia and chronic inflammation.