The world has been colonized by microbes. They are everywhere, even in many places in the human body. New research has suggested however, that the healthy human bloodstream does not host a stable microbiome. There are microbes that can be found in blood, however, and knowing more about them could help improve the use of donated blood and reduce the risk of infections during transfusion. While this study used samples from healthy individuals, it may one day be possible to use blood microbes as a kind of diagnostic indicator. The findings have been reported in Nature Microbiology.
In a sterile environment, there are no microbes, and human blood has generally been thought of as sterile. Microbes can get into the bloodstream through cuts, or after we brush our teeth, but the immune system usually takes care of those microorganisms, eliminating them before they cause harm.
In recent years, however, there has been speculation that there is a blood microbiome, said lead study author and graduate student Cedric Tan of the UCL Genetics Institute and Francis Crick Institute. "Here, we have confirmed this is not the case, as most people's blood does not contain microbes, and the microbial species found in some people's blood varied substantially between individuals."
While the human gut microbiome also varies considerably from one individual to another, there are a few species that tend to be found in most healthy human microbiomes, and they are very abundant.
In this study, the researchers assessed sequencing data from a project of the National Precision Medicine program in Singapore, called SG10K Health. It included information from 9,770 people.
Contamination can present a problem during microbiome studies, because sterility is so challenging to maintain. The study authors took steps to remove this contamination from the data, and then determined that microbes are truly detected in blood only on rare occasions, and they are not a regular component of blood; bacteria only appears sporadically in blood.
The blood samples from about 84 percent of individuals represented in this work did not show any sign of microbes, and fewer than 5 percent of people shared the same species.
But the research also suggested that in some healthy people's blood, there are bacteria that are replicating. These microbes are usually found in the skin, oral, or gut microbiomes, which may indicate that microbes are occasionally able to move into the bloodstream without harming people.
While the study concluded that there is not a basic set of microbial species living the blood of healthy individuals, there are some microbes that can be found there. Knowing more about these species could also help us compare the microbes in healthy human blood to species that might be found in people with various diseases.
This research was focused on healthy people, the study authors noted, and we don't yet know whether in a disease state such as cancer or diabetes, the situation may be different. Future studies could investigate the possibility that blood microbial communities are altered and potentially correlate with the progression or severity of disease, they added.
Sources: University College London, Nature Microbiology
