Virus-like ‘Obelisks’ in Human Gut and Mouth: A Mysterious Find

Scientists have discovered a new class of virus-like entities hiding in the human microbiome. These ‘Obelisks’ may influence the gene activity of their bacterial hosts.

What are Obelisks?

Obelisks are tiny loops of RNA, a genetic cousin of DNA, that can replicate inside other cells. They are similar to viroids, which are known to infect plants, but they have some unique features. They are named Obelisks because their RNA folds into a thin rod-like shape.

Obelisks differ from larger, RNA-based viruses in several ways. First, they are naked, lacking the protective shells that viruses use to hold their genetic material. Second, their RNA does not contain instructions to build proteins; whereas viruses carry instructions for their outer shells and for certain enzymes they need to replicate, Obelisks co-opt these enzymes from their hosts.

How were they discovered?

A team of researchers from Stanford University and other institutions searched for possible Obelisks among the genes of microbes that reside in the human body. They used previously published data on gene activity in different microbial communities within the body, known as metatranscriptomes.

They found that Obelisks were present in roughly 7% of the metatranscriptomes from human feces, which give a snapshot of gene activity in the gut microbiome. They also found them in 17 out of 32, or about 53%, of the mouth metatranscriptomes they screened.

In a report published on Jan. 21 to the preprint database bioRxiv, the team introduced ‘Obelisks’, a newly named class of viroid they discovered in the human gut and mouth. In all, they identified nearly 29,960 examples of the viroids. (The work has not yet been peer-reviewed or published in a scientific journal.)

What do they do?

The researchers confirmed one host for these Obelisks, namely, a common bacterium found in the mouth called Streptococcus sanguinis. They suspect that at least some of the other Obelisks may also infect bacteria.

The researchers do not know yet what effects these Obelisks have on their hosts or on the human body. They may influence the gene activity within the human microbiome, which plays a role in various aspects of health and disease.

Interestingly, some of the newfound Obelisks seemed to contain instructions for enzymes needed for replication, making them more complex than viroids that had been described previously.

The discovery of Obelisks opens up new questions about the diversity and evolution of these virus-like entities and their interactions with their hosts and environments.