A study discovered no common blood microorganisms in healthy persons.

A new study led by a UCL researcher found that there is no stable microbial community living in the bloodstream of healthy humans.

Given that blood donations are an essential component of medical care, the new Nature Microbiology paper provides significant confirmation. By reducing the likelihood of transfusion-related infections, better microbial tests for blood donations may be developed as a result of our increased understanding of the types of microbes that can be present in blood.

principal author, Ph.D. Human blood is typically regarded as sterile, according to student Cedric Tan (UCL Genetics Institute and Francis Crick Institute). Even though microorganisms can occasionally enter the bloodstream through wounds or after brushing your teeth, the immune system usually quickly clears this up.

“Human blood is often regarded as sterile. While bacteria may enter the circulation through a wound or after cleaning one’s teeth, the immune system usually eliminates them rapidly.”

Lead author, Ph.D. student Cedric Tan (UCL Genetics Institute and Francis Crick Institute)

“However, in more recent years, there has been speculation that the blood may contain a community of microbes, challenging this paradigm. This is not the case, as shown by the fact that the majority of people’s blood is free of microbes and that the microbial species that were occasionally found in the blood varied greatly from person to person.

Cedric collaborated on the study’s population-scale sequencing data analysis with a group led by Dr. Niranjan Nagarajan at A*STAR’s Genome Institute of Singapore. “SG10K Health” is a project of Singapore’s National Precision Medicine Program.

The team discovered that microbes were only infrequently and sporadically detected in blood rather than existing as stable communities after accounting for contamination that is common in microbiome investigations. Less than 5% of the 9,770 participants in their sample shared the same species of microbe, and 84% had no microbes at all.

The majority of these bacteria are typically found in the human gut, oral, or skin microbiome, and the researchers also discovered evidence that some of these bacteria may be replicating in the blood of healthy people. Their research suggests that although there isn’t a core group of species that colonize the blood of healthy people, microbes occasionally enter the bloodstream from other body sites without spreading disease.

The findings also offer a useful resource for the kinds of microbes that one might occasionally anticipate finding in the blood of healthy humans. Characterizing the variety of microbial species present in healthy individuals’ blood provides a crucial baseline for comparison with that of diseased individuals, shedding light on the potential correlation between healthy individuals’ blood microbial profiles and health status.

Cedric stated in a research briefing that the presence of blood microbial communities in patients with chronic diseases like cancer or diabetes is not excluded by the study’s focus on healthy people. The next logical step would be to find out if blood-borne communities are present in these cases and see if they are connected to the severity and development of the disease. This strategy may create new opportunities for the treatment of chronic illnesses using the microbiome.

More information: Cedric C. S. Tan et al, No evidence for a common blood microbiome based on a population study of 9,770 healthy humans, Nature Microbiology (2023). DOI: 10.1038/s41564-023-01350-w

No microbial community in the blood of healthy individuals, Nature Microbiology (2023). DOI: 10.1038/s41564-023-01364-4

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