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via Sci News :  A new survey of DNA fragments circulating in human blood suggests the bacteria and viruses living within us are vastly more diverse than previously known. In fact, more than 50% of that DNA has never been seen before.

“We found things that are related to things people have seen before, we found things that are divergent, and we found things that are completely novel,” said senior author Professor Stephen Quake, of Stanford University.

Professor Quake and colleagues collected and analyzed samples from 188 patients (156 heart, lung and bone marrow transplant recipients, and 32 pregnant women).

Of all the non-human DNA fragments the team gathered, 52% failed to match anything in existing genetic databases.

“Through massive shotgun sequencing of circulating cell-free DNA from the blood of 1,351 independent samples, we identified hundreds of new bacteria and viruses which represent previously unidentified members of the human microbiome,” the authors said.

“Analyzing cumulative sequence data from the blood samples enabled us to assemble 7,190 contiguous regions larger than 1 kbp, of which 3,761 are novel with little or no sequence homology in any existing databases.”

The ‘vast majority’ of the mystery DNA belonged to a phylum called Proteobacteria, which includes, among many other species, pathogens such as Escherichia coli and Salmonella.

Previously unidentified viruses in the torque teno virus (TTV) family, generally not associated with disease but often found in immunocompromised patients, made up the largest group of viruses.

“We’ve doubled the number of known viruses in that family through this work,” Professor Quake said.

“Perhaps more important, we’ve found an entirely new group of TTVs.”

“Among the known TTVs, one group infects humans and another infects animals, but many of the ones we found didn’t fit in either group.”

“We’ve now found a whole new class of human-infecting ones that are closer to the animal class than to the previously known human ones, so quite divergent on the evolutionary scale,” he added.

The research is published in the Proceedings of the National Academy of Sciences.


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