WashU Medicine researchers have rewritten the story on a group of viruses once thought to directly infect human tissue and play a role in gastrointestinal disease. They found that the common virus actually infects gut bacteria, redefining how these viruses impact human health.
Image Credit: Sara Moser/WashU Medicine
Scientific Frontline: Extended "At a Glance" Summary: Picobirnaviruses
The Core Concept: Picobirnaviruses are a prevalent group of RNA viruses within the human gut microbiome that act as bacteriophages, specifically infecting bacterial cells rather than human tissue.
Key Distinction/Mechanism: Unlike most stomach viruses that directly attack human cells to cause illness, picobirnaviruses multiply inside gut bacteria under oxygen-free conditions, meaning their association with gastrointestinal diseases stems from their disruption of the bacterial ecosystem.
Origin/History: First discovered nearly 40 years ago in stool samples from patients with diarrhea, these viruses were widely presumed to be human pathogens until an October 2026 study in Nature Microbiology revealed their true bacterial host.
Major Frameworks/Components:
- Viral Replication Environment: The viruses actively multiply in oxygen-free laboratory models populated by human gut bacteria, ceasing replication when exposed to antibiotics.
- Bacterial Host Identification: Using advanced molecular imaging, scientists identified the virus inside Candidatus Anaerotignum faecalis, a newly classified gut bacterium.
- Viral Classification: The discovery reclassifies the Picobirnaviridae family, establishing them as only the third known clade of RNA bacteriophages.
- Disease Pathology: Although not directly infecting human cells, picobirnaviruses may contribute to clinical conditions like acute diarrhea, Type 1 diabetes, and graft-versus-host disease by destroying beneficial microbes and triggering inflammation.
Branches of Science: Virology, Molecular Microbiology, Gastroenterology, and Genomics.
Future Application: Understanding these RNA bacteriophages paves the way for targeted microbiome therapies, potentially harnessing the viruses to shape bacterial communities in the gut and treat gastrointestinal or microbiome-linked diseases.
Why It Matters: This paradigm shift fundamentally alters our understanding of the gut ecosystem, redirecting the focus of disease research from direct viral infection of human cells toward the complex interactions between viruses and bacterial communities.
Thriving alongside trillions of bacteria in the human digestive tract is a vast population of viruses. Ever since one such denizen of the gut microbiome, known as the picobirnavirus, was first spotted decades ago, researchers thought it directly infected human tissue and played a role in gastrointestinal illness. But researchers at Washington University School of Medicine in St. Louis and collaborators have just rewritten that story: this widespread virus infects gut bacteria, not human cells.
The findings published in Nature Microbiology, are a major milestone for the field of virology and redefine picobirnaviruses as only the third known family of bacteria-infecting RNA viruses ever discovered.
“This discovery changes our understanding of the gut ecosystem,” said David Wang, the Robert C. Packman Professor at WashU Medicine. “RNA viruses that infect bacteria in the human body were thought to be rare. But picobirnaviruses are abundant in the gut, and knowing that they infect bacteria is a first step in enabling us to better understand what role they play in causing disease.”
Stomach bug or bacteria hunter?
For nearly four decades, scientists thought they had identified another human stomach bug.
Picobirnaviruses were first described in stool samples from patients suffering from diarrhea, turning up alongside notorious culprits such as rotavirus and norovirus. Without the tools to grow the virus in the laboratory, the researchers assumed the newcomer was directly attacking human cells to cause gastrointestinal misery.
That is, until advances in sequencing technology led the Wang lab to unexpectedly find traces of picobirnavirus genetic material while studying Caenorhabditis elegans, a microscopic roundworm often used in genetics research. Although humans and worms differ in many ways, one commonality is bacteria: worms eat them, and humans carry trillions of them in their gut.
The researchers asked: Do picobirnaviruses infect bacteria?
First author Tianyu Gan, a postdoctoral research fellow in Wang’s lab, in collaboration with researchers at Cambridge University Hospitals NHS Foundation Trust, Flinders University, and San Diego State University, set out to find the virus inside a bacterial host. He introduced human stool samples to microbe-free mice, establishing the first laboratory model where the virus could actively multiply. Next, Gan and his team harvested the viral population from the mouse gut and grew it in lab dishes filled with gut bacteria under oxygen-free conditions. Because mammalian cells cannot survive without oxygen, the virus’s growth over 20 days suggested it was multiplying inside bacteria. Antibiotics that killed off the bacteria also halted the virus from multiplying.
Using advanced molecular imaging and cellular sorting, the researchers found picobirnaviruses inside a newly identified gut bacterium they named Candidatus Anaerotignum faecalis.
The presence of picobirnaviruses in the gut has been linked to clinical conditions including acute diarrhea, graft-versus-host disease (a complication of stem cell transplants), and type 1 diabetes.
It’s possible that, although the picobirnavirus doesn’t infect human cells directly, it could be destroying beneficial gut bacteria. Wiping out friendly microbes disrupts the gut ecosystem, allowing harmful pathogens to bloom and prompting bacteria to release toxins. Moreover, human gut cells can absorb viral particles and spark inflammation without the virus multiplying inside human tissue.
On the other hand, detecting high levels of picobirnavirus in patients with these conditions does not prove the virus causes disease, Wang warned. Even if it is not a pathogen, it could serve as a biomarker, signaling gut distress.
“The first step to understanding how a virus impacts human health is understanding what it infects,” Wang said. “Knowing that picobirnaviruses infect bacteria shifts our focus to the microbiome. It opens new opportunities to study how these viruses shape bacterial communities in the gut and how we might harness them for targeted therapies.”
Reference material: What Is: Bacteriophages
Funding: This research was supported by the National Institute for Health and Care Research Biomedical Research Centre: Cambridge, grant number NIHR203312; the Stephen I. Morse Fellowship; and the National Institutes of Health (NIH), grant numbers R01AI165915, RC2DK116713, and R01AI193051.
Disclaimer: The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.
Published in journal: Nature Microbiology
Authors: Tianyu Gan, Anne L. Rosen, Leran Wang, Susan Vernon, Lindsay Droit, Amanda N. Pinski, Guang Wu, Luis A. Chica Cardenas, Kathie A. Mihindukulasuriya, Anne Paredes, Zaireen Pios, Megan T. Baldridge, Joanna E. Rimmer, Miles Parkes, Elizabeth A. Dinsdale, Robert A. Edwards, Anca M. Segall, Scott A. Handley, Andrew L. Kau, and David Wang
Source/Credit: Washington University School of Medicine in St. Louis | Marta Wegorzewska
Edited by: Scientific Frontline
Reference Number: vi103026_02
