For almost forty years, doctors suspected that a tiny virus found in people’s stool might be making them sick. It turns out the picobirnavirus was never interested in human cells at all. A team led by David Wang at WashU Medicine in St. Louis reports in Nature Microbiology, published on October 29, that the virus lives off bacteria inside our intestines.
The bug first showed up in samples from patients with diarrhea, often alongside well-known culprits like rotavirus and norovirus. Nobody could grow it in a dish, so researchers assumed it attacked the lining of the gut directly. The clue that changed the picture came from an unexpected place: Wang’s lab found picobirnavirus genes while studying tiny roundworms, animals whose diet consists of bacteria.
Postdoctoral researcher Tianyu Gan, the study’s first author, then gave germ-free mice human stool and watched the virus multiply, the first lab model where that worked. He moved the virus into cultures of gut bacteria kept without oxygen. Mammalian cells die in such conditions, so steady viral growth over 20 days pointed straight at a bacterial host, and antibiotics that wiped out the bacteria also stopped the virus. Imaging finally caught the virus inside a newly described microbe the team named Candidatus Anaerotignum faecalis.
Only two other RNA virus groups were previously known to target bacteria, so picobirnaviruses now join a very short list. “This discovery changes our understanding of the gut ecosystem,” Wang said. The finding does not clear the virus entirely. By killing helpful microbes, it could let harmful ones flourish, and gut cells may still react with inflammation when they swallow viral particles.
Still, Wang warns, finding lots of the virus in a sick person is no proof that it caused the illness. The virus might instead serve as a marker that something in the gut has gone wrong.
