When an individual bacterial virus grows in a bacterial host suspended in a top agar lawn, its progeny infect and lyse the surrounding host cells. This causes the appearance of a "hole" or plaque in the otherwise homogeneous bacterial lawn. Since each plaque represents a single virus, the number of viruses in the aliquot added to the plate is equal to the number of plaques which appear.

Protocol: Titering of Bacterial Viruses Protocol