Bizarre Virus-like ‘Obelisks’ Found in Human Mouths and Guts

Our mouths and guts are teeming with mysterious somethings that are unknown to science, new research suggests. A team says they’ve discovered distinct virus-like structures hanging out among the bacteria that live in our bodies. The researchers have coined these structures “obelisks,” and they might further redefine what it means to be a living thing.

Viruses are small packages of genetic material made of either DNA or RNA enclosed inside a protective protein shell. They’re characterized by their inability to make more of themselves without the help of other, more complex organisms, whether it’s bacteria or animal cells. Scientists still argue over whether this inherently parasitic attribute disqualifies viruses from being considered a form of life.

Over time, we’ve discovered that there are microscopic entities that appear to be even stranger than a traditional virus. Satellite viruses have a protein shell but can’t replicate inside a cell without a second helper virus, for instance (some will even “bite” onto their helper so both enter the host at the same time). And there are other agents that have no protein shell at all and are simply circular loops of RNA, which we’ve called viroids.

To date, viroids have only conclusively been found in plants. But some recent studies have indicated that similar agents infect other forms of life, too. Researchers at Stanford University say they’ve now found compelling evidence of these agents inside the microbial communities of bacteria that line our mouths and guts (these tiny neighborhoods are also known as microbiomes).

The scientists analyzed databases that categorized the active genes of gut and mouth microbes, using an algorithm to look for unknown genetic sequences that could represent independent viroid-like loops of RNA. They identified almost 30,000 of these unique sequences, dubbed obelisks, within the databases, with examples from across the world. All told, the obelisks were found in about 7% of the gut bacteria and half of the mouth bacteria microbiomes analyzed by the researchers. And they were able to find evidence of a specific bacterial species (Streptococcus sanguinis) playing host to a specific population of obelisks.

“As such, obelisks comprise a class of diverse RNAs that have colonized, and gone unnoticed in, human, and global microbiomes,” the authors wrote in their preprint paper, released on bioRxiv this month.

The team’s findings are preliminary, so they should be viewed with added caution for the time being. But if their work is validated by other research, then they’ve uncovered yet another hidden layer of the microbial world—one with so many questions left unanswered about it.

The authors say that the predicted characteristics of obelisks don’t really line up with either viroids or other subviral agents, for instance. And they seem to carry genes that have never been seen in any other organisms. We also don’t know if and how they might affect human cells or our health in general. And we’re still unclear about how all these things came to “exist” in the first place. Some scientists argue that viroids could be directly related to the ancestors that evolved into viruses, but it’s also possible that viroids used to be viruses that shed their more complex machinery, or maybe today’s viroids are a mix of both.

“I think this [work] is one more clear indication that we are still exploring the frontiers of this viral universe,” Simon Roux, a scientist at the DOE Joint Genome Institute at Lawrence Berkeley National Laboratory who is unaffiliated with the research, told Science Magazine.