A bacterial symbiont represses replication of giant viruses in amoebae

The discovery of giant viruses changed our perception of the viral world. With genome and particle sizes comparable to those of bacteria and a number of cellular features, such as nearly complete sets of translation-related genes, giant viruses sparked a heated debate about the evolutionary origin of these unusual viruses of protists and other unicellular microbes. Host-switches and competition with intracellular bacterial symbionts were proposed as important drivers of giant virus evolution. Yet, bacteria-virus interactions in protists are poorly understood. Here, we studied the role of bacterial symbionts of free-living amoebae in the establishment of giant virus infections. To investigate these interactions in a system that would be relevant in nature, we first isolated a giant virus and potential amoeba hosts infected with bacterial symbiont from the same environmental sample. Subsequently, co-infection experiments were carried out, using the fresh environmental isolates as well as additional amoeba lab strains.