Bacteria are a major determinant of Orsay virus transmission and infection in .

The microbiota is a key determinant of the physiology and immunity of animal hosts. The factors governing the transmissibility of viruses between susceptible hosts are incompletely understood. Bacteria serve as food for and represent an integral part of the natural environment of . We determined the effects of bacteria isolated with from its natural environment on the transmission of Orsay virus in using quantitative virus transmission and host susceptibility assays. We observed that species promoted Orsay virus transmission, whereas MYb11 attenuated virus transmission relative to the standard laboratory bacterial food OP50. We found that pathogenic strains PA01 and PA14 further attenuated virus transmission. We determined that the amount of Orsay virus required to infect 50% of a population on MYb11 compared with MYb71 was dramatically increased, over three orders of magnitude. Host susceptibility was attenuated even further in the presence of PA14. Genetic analysis of the determinants of required for attenuation of susceptibility to Orsay virus infection revealed a role for regulators of quorum sensing. Our data suggest that distinct constituents of the microbiota and potential pathogens can have widely divergent effects on Orsay virus transmission, such that associated bacteria can effectively determine host susceptibility versus resistance to viral infection. Our study provides quantitative evidence for a critical role for tripartite host-virus-bacteria interactions in determining the transmissibility of viruses among susceptible hosts.