A human gut-associated picobirnavirus establishes the Picobirnaviridae as the third clade of RNA bacteriophages

Picobirnaviruses (PBVs) are commonly detected RNA viruses in human and animal guts and are associated with multiple human diseases. Although PBVs have been assumed to infect vertebrate hosts, there is neither direct evidence for this nor any culture system to propagate PBVs. Here we demonstrate that a PBV infects bacteria and is therefore an RNA phage. Two different PBVs from human faeces were propagated in germ-free mice after faecal microbiota transplantation, establishing a mouse model for PBV. One PBV could be cultured in vitro, in a community of anaerobic gut bacteria, and its growth could be inhibited by multiple antibiotics. RNA fluorescence in situ hybridization revealed PBV RNA within bacterial cells, and enrichment and sequencing of PBV RNA-containing cells identified a previously unidentified 'Candidatus Anaerotignum faecalis' as its host. Our findings define the family Picobirnaviridae as only the third clade of RNA phages in addition to leviviruses and cystoviruses.