40-year mystery solved: Common gut virus infects bacteria, not human cells

Picobirnaviruses have been detected in human stool for nearly 40 years and were long suspected of infecting human tissue and contributing to diarrhea. Researchers at Washington University and collaborators now show that these widespread gut viruses instead infect bacteria, making them only the third known group of RNA viruses that target bacteria. The question arose after viral genetic material turned up in research involving a roundworm that, like humans, lives in close association with bacteria. To test the idea, the team introduced stool samples into germ-free mice, then grew virus from the mice in oxygen-free cultures containing gut bacteria. The virus multiplied in those cultures, but stopped when antibiotics killed the bacteria. Imaging and cell-sorting experiments located the virus inside a previously unidentified gut bacterium, named Candidatus Anaerotignum faecalis.

The finding redirects research toward how picobirnaviruses may affect the gut microbiome rather than by directly multiplying in human cells. The virus could harm beneficial bacteria, disrupt microbial balance, or contribute indirectly to inflammation; gut cells may also absorb viral particles without supporting viral replication. Picobirnavirus has been associated with diarrhea, graft-versus-host disease and Type 1 diabetes, but those links do not show that it causes these conditions. It may instead be a sign of gut disturbance. Identifying its bacterial host provides a basis for studying its effects on microbial communities and possible therapeutic uses, while clarifying that its role in human health remains unresolved.