Antiviral strains associate with endoplasmic reticulum membranes and induce lipid droplet formation to restrict dengue virus replication.

are a genus of insect endosymbiotic bacteria which includes strains Mel and AlbB that are being utilized as a biocontrol tool to reduce the incidence of -transmitted viral diseases like dengue. However, the precise mechanisms underpinning the antiviral activity of these strains are not well defined. Here, we generated a panel of -derived cell lines infected with antiviral strains Mel and AlbB or the non-antiviral strain Pip to understand host cell morphological changes specifically induced by antiviral strains. Antiviral strains were frequently found to be entirely wrapped by the host endoplasmic reticulum (ER) membrane, while Pip bacteria clustered separately in the host cell cytoplasm. ER-derived lipid droplets (LDs) increased in volume in Mel- and AlbB-infected cell lines and mosquito tissues compared to cells infected with Pip or -free controls. Inhibition of fatty acid synthase (required for triacylglycerol biosynthesis) reduced LD formation and significantly restored ER-associated dengue virus replication in cells occupied by Mel. Together, this suggests that antiviral strains may specifically alter the lipid composition of the ER to preclude the establishment of dengue virus (DENV) replication complexes. Defining 's antiviral mechanisms will support the application and longevity of this effective biocontrol tool that is already being used at scale.IMPORTANCE transmits a range of important human pathogenic viruses like dengue. However, infection of with the insect endosymbiotic bacterium, , reduces the risk of mosquito to human viral transmission. is being utilized at field sites across more than 13 countries to reduce the incidence of viruses like dengue, but it is not well understood how induces its antiviral effects. To examine this at the subcellular level, we compared how different strains of with varying antiviral strengths associate with and modify host cell structures. Strongly antiviral strains were found to specifically associate with the host endoplasmic reticulum and induce striking impacts on host cell lipid droplets. Inhibiting -induced lipid redistribution partially restored dengue virus replication demonstrating this is a contributing role for antiviral activity. These findings provide new insights into how antiviral strains associate with and modify host cells.