Asian Scientist Magazine (Jun. 17, 2023) — Microorganisms in our gut and skin play a crucial role in shaping our health. But the existence of a stable microbial community within our bloodstream has long been a subject of scientific debate. Researchers from Singapore have recently published a study confirming the absence of a ‘blood microbiome’. The study was published in Nature Microbiology.
Human blood was traditionally believed to be sterile but recent studies have challenged this idea by suggesting the presence of a steady microbial population in the blood of healthy individuals. But these claims remained controversial largely due to the small sample sizes of the studies and inadequate checks for contamination. More importantly, it remained uncertain whether these detected microbes originated from other parts of the body, whether there was a core set of shared microbes among the population, and if our health was linked to the structure and function of this microbial community.
In order to out the debate to rest, scientists from the Genome Institute of Singapore of the Agency for Science, Technology and Research (A*STAR), performed the largest population study that analyzed the blood DNA sequencing data from 9,770 healthy individuals. The researchers used a set of filters to account for potential contamination during sample handling and testing to distinguish true microbial signals in the blood.
“Our laboratory has a long-term experience with microbiome analyses and controlling contaminants or artefacts in sequencing data. The lab continues to explore how different fungi, bacteria and viruses co-exist peacefully at various body sites and how they play a role in shaping human health,” said Chia Minghao, Senior Research Fellow at A*STAR’s GIS and co-supervising author of the study.
The team found that in healthy human blood, the presence of microbes was both rare and random, with no evidence of organized communities. Out of the 117 microbial species detected by the team, the majority were identified to have transited from other body sites. While analyzing microbial DNA signatures, the scientists also identified certain bacteria showing signs of active replication. However, they proposed that these microbes likely replicated in other body locations before entering the bloodstream. This aligned with the fact that the presence of these transient microbes did not negatively affect the health status of the participants.
“We have known for decades that microbes can invade the bloodstream and cause disease — the first report of bacteria in human blood dates back to around 1969. However, in medical practice, the blood is generally considered sterile, with transient migration of bacteria into the bloodstream, for example during daily routine activities like brushing teeth. Over the past few years, scientists began suggesting the presence of a microbial community in blood, challenging this. Our findings refute the claims by these recent studies,” said Niranjan Nagarajan, lead author of the study and Senior Group Leader at A*STAR’s GIS’ Laboratory of Metagenomic Technologies and Microbial Systems and Associate Director of Genome Architecture.
Although this study was the most comprehensive one to challenge the concept of a ‘blood microbiome,’ the scientists said that there is still room for improvement. Some of the identified microbial species were still flagged as being of environmental or non-human origin. Nonetheless, even considering the likelihood of residual contamination, the study did not find a shared microbiome across healthy individuals.
This discovery is particularly important for establishing a crucial baseline for studying bloodstream infections and for developing better microbial testing in blood donations. By doing so, it could help minimize the risk of transfusion-related septic shock or death in patients.
Professor Patrick Tan, Executive Director of A*STAR’s GIS, said, “This is the largest analysis to date in the world, of microbial signatures in human blood. These findings by Dr. Niranjan Nagarajan’s team are an important move toward more informed decision-making in medical therapeutics.”
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Source: Genome Institute of Singapore ; Image: Shutterstock
The article can be found at No evidence for a common blood microbiome based on a population study of 9,770 healthy humans.
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