Our bodies are home to trillions of microorganisms, including bacteria, viruses, fungi, and a host of other microorganisms. Now, scientists at Stanford University have discovered a new class of biological entities inside our bodies, which they have named "Obelisks."

The microbiome we carry around with us is huge, and new things are still being learned about its makeup and how it affects our health. We often discover new strains of bacteria or viruses in the microbiome, but it's rare that scientists discover an entirely new set of entities that doesn't fit into any known category.

The Stanford team calls them "obelisks" thanks to their rod-like structure. We are all familiar with the former, while viruses are simpler RNA molecules that can replicate by breaking down and reassembling their genomes, but do not produce proteins and have no protective shell. Obelisks have the basic structure of a virus, but like viruses, their simple genomes appear to encode unknown proteins that scientists call "obelisks."

It turns out that obelisks are very common and come in a surprising variety. Scientists found nearly 30,000 different types of obelisks in microbiome samples from more than 400 people around the world. They were found in about 50% of the oral microbiome samples tested and 7% of the intestinal samples. To date, they appear to have remained undiscovered, as they don't look like anything else we know of.

"We find that obelisks form their own unique phylogenetic group with no detectable sequence or structural similarity to known biological agents," the researchers wrote in their paper.

What exactly they do in our bodies remains a mystery. They may help or harm their host, which may not be us but bacteria or fungi that call our bodies home. So far, the leading candidate is Streptococcus sanguis, which is found in dental plaque.

Streptococcus sanguis lives in the human oral cavity and is host to a newly described group of RNA entities. Image source: UK Health Safety Authority/Science Photo Library

The researchers say this easily cultured bacterial species would be the best starting point for further studies of the obelisks.

The study has not yet been peer-reviewed, but has been posted as a preprint on bioRxiv.