What is already known
Although sometimes microbes can enter the bloodstream through a wound or after tooth-brushing, the human blood is generally considered sterile. However, recent studies have suggested that several microbial species may circulate in the blood of healthy people.
What this research adds
Researchers analyzed blood samples from nearly 10,000 people and found that most of them did not have any microbes in their bloodstream. Less than 5% of study participants shared the same microbial species, which are typically found in the gut, oral or skin microbiotas.
The findings suggest that microbes can occasionally enter the bloodstream from other body sites without causing disease, but they do not support the idea of a common blood microbiota.
Recent studies have suggested that several microbial species may circulate in the blood of healthy people. But a new analysis of blood samples from nearly 10,000 individuals does not support the idea of a common blood microbiota.
The study, published in Nature Microbiology, identified the types of microbes that may be occasionally observed in the blood of healthy people — work that may help to develop better microbial tests in blood donations to reduce the risk of transfusion-related infections.
Although sometimes microbes can enter the bloodstream through a wound or after tooth-brushing, the human blood is generally considered sterile. Most of the studies that have proposed the existence of a microbiota in the blood of healthy people were either small in size or lacked rigorous checks to rule out contamination.
To test the idea of a microbial community in healthy human blood, Cedric Tan at University College London, Niranjan Nagarajan at the National University of Singapore and their colleagues analyzed blood samples from 9,770 healthy people.
After accounting for contamination, the researchers identified 117 microbial species in the blood samples. Most of these microbes are typically found in the gut, oral or skin microbiotas, and they were different from pathogens found in blood cultures from hospitals.
The most common microbial species in blood samples was Cutibacterium acnes, a pathogen related to the skin condition of acne. The team found C. acnes in 4.7% of individuals — a finding that refutes the existence of common ‘core’ species across healthy individuals.
More than 80% of the samples did not yield any microbes after decontamination, and the rest yielded about one microbial species per sample. “Our results suggest that the presence of microbes in the blood of healthy individuals is infrequent and sporadic,” the researchers say.
No blood microbiota
The researchers also found evidence of replicating bacteria in the blood of healthy people, but they did not detect any microbial associations that are seen in other microbial communities.
The findings suggest that bacteria are detected in blood only on rare occasions, and they are not a regular component of blood. “On the basis of these findings, we advocate against the use of the terms ‘blood microbiome’ or ‘circulating microbiome’, which are potentially misleading when referring to the detection of microbial DNA or of microbial cells in blood due to transient translocation events,” the researchers say.
Future research should determine if bacteria are present in the blood of people with chronic conditions such as cancer or diabetes, and whether the microbes are associated with disease severity and progression. This approach, the researchers say, could help develop microbiota-based interventions for chronic diseases.