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Hundreds of new bacteria and viruses identified in human blood samples

Researchers from Stanford University have made some unexpected discoveries when they investigated less invasive ways to predict whether a patient's body would reject a transplanted organ. They found that of the non-human DNA in blood plasma, hundreds of new bacteria and viruses didn't match anything in existing genetic databases. The results indicate that the human microbiome is even far more diverse than once suspected.

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Microbiota versus microbiome

The human microbiota refers to the populations of micro-organisms that live on or in the human body, including bacteria, fungi, viruses and other single-celled organisms. All of the genes inside these microbes constitute the microbiome. With the help of advanced high-throughput DNA sequencing techniques, research over the past decade has characterized many previously unknown species. Research projects such as the Human Microbiome Project and MetaHIT have characterized many previously unknown taxa. However, these projects targeted specific niches such as the gut and the skin and therefore did not detect organisms residing in other body sites. The Stanford study showed for the first time that other niches accessed by blood plasma contain a substantial amount of bacterial species, of which hundreds were previously unidentified.

The study

The researchers discovered more or less by accident that we still not know the majority of our microbes. The team was investigating non-invasive ways to predict whether an organ transplant’s immune system would reject or accept a new organ. They hoped that they could tell from the DNA present in blood samples whether a transplant is successful. Blood cells contain, besides human DNA, also DNA from microbes at other places in our body, also called circulating cell-free DNA (cfDNA). Higher amounts of cfDNA from the donor in the blood of the recipient correlates with an increased risk of transplant failure. The researchers analysed the cfDNA of 1351 samples form 188 patients through massive shotgun sequencing in 4 longitudinally cohorts. They discovered that 99% of all the found non-human DNA fragments failed to math anything in existing genetic databases. 

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             Cell-free DNA. Source: National Human Genome Research Institute


The vast majority of this newly discovered DNA belonged to a phylum called proteobacteria, which includes, among many other species, pathogens such as E. coli and Salmonella. In addition, they found numerous novel phages and previously unidentified viruses in the Torque Teno family, generally not associated with disease but often found in immunocompromised patients.

These newly discovered uncharacterized microbes have potential consequences for our health. They may cause chronic diseases that, to date, have unknown etiology. With these results, scientist can study a range of new viruses and bacteria which can help doctors manage and track outbreaks. 



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Hundreds new microbes discovered

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