About half the world’s population carries a newly discovered gut virus, dubbed CrAssphage, says a team of virologists led by Prof Robert Edwards of San Diego State University.

CrAssphage is a bacteriophage (also known as phages or bacterial viruses), a member of a group of viruses that infect bacteria.

Prof Edwards and his colleagues named this virus after the Cross-Assembly (CrAss) software program used to discover it.

Interestingly, CrAssphage was discovered entirely by accident.

While sifting through data from previous studies on gut-inhabiting viruses, the virologists noticed an unusual cluster of viral DNA – about 97,000 base pairs long.

When they checked this discovery against a comprehensive listing of known viruses, they came up empty.

They then screened for CrAssphage across the database of the NIH’s Human Microbiome Project, and Argonne National Laboratory’s MG-RAST database, and again found it in abundance in samples.

To prove that CrAssphage they discovered in their data actually exists in nature, the researchers used DNA amplification technique to locate the virus in the original samples used to build NIH’s database.

“So we have a biological proof that the virus they found with the computer actually exists in the samples. This was a new virus that about half the sampled people had in their bodies that nobody knew about,” said Dr John Mokili of San Diego State University, who is a co-author of the paper describing the discovery in the journal Nature Communications.

The fact that CrAssphage is so widespread indicates that it probably isn’t a particularly young virus, either.

“As far as we can tell, it’s as old as humans are. We’ve basically found it in every population we’ve looked at,” Prof Edwards said.

It’s unknown how CrAssphage is transmitted, but the fact that it was not found in very young infants’ fecal samples suggests that it is not passed along maternally, but acquired during childhood.

The makeup of the viral DNA suggests that it’s circular in structure.

Further work has confirmed that the viral DNA is a singular entity, but it’s proven difficult to isolate.

According to the scientists, CrAssphage infects one of the most common types of gut bacteria, Bacteroidetes.

These bacteria live toward the end of the intestinal tract, and they are suspected to play a major role in the link between gut bacteria and obesity.

What role crAssphage plays in this process will be a target of future research.

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Dutilh, B. E. et al. 2014. Unknown sequences in faecal metagenomes reveal a widely distributed and highly abundant bacteriophage. Nat. Commun. 5: 4498; doi: 10.1038/ncomms5498