Figure 2. DIP-Mediated Interference with Infectious Virus Propagation Can Be Visualized by Plaque Assay.

Pictured is a representative plaque assay performed on a laboratory stock of LCMV strain Armstrong 53b. The highest concentration of infectious virus was added to the far left well and the wells to the right received successive serial 10-fold dilutions of the initial inoculum. Each well was stained with crystal violet and the image converted to gray scale (i.e., black staining indicates intact areas of the Vero E6 monolayer, while white areas/foci represent LCMV plaques). Starting from the left, note that despite receiving the most infectious virus, the first three dilutions show no or few plaques. This is presumably because all of the available cells were infected with DIPs that blocked propagation of any standard infectious particles that also infected those cells. Only by the fourth and fifth serial dilutions are individual plaques visible, presumably because DIPs were sufficiently diluted to allow standard infectious particles to productively infect cells in the absence of DIPs and thus create plaques. Abbreviations: DIPs, defective interfering particles; LCMV, lymphocytic choriomeningitis virus.