Viruses are inert outside of a host cell. Viruses are cultured in the laboratory using cells of the appropriate host. Animal viruses are grown in tissue culture, on a monolayer or a suspension of cells. Bacterial viruses are grown in soft agar or in broth seeded with the host cells.
Lytic viruses lyse the cell in order to leave.
Thus they can be enumerated by making dilutions of the virus and assaying on a seeded agar surface for plaque formation. Plaques are clear areas on a lawn of host cells that represent the point at which a single infectious virus particle was deposited. As a result of subsequent lytic cycles, the host cells in the region are destroyed. The size of the plaque is dependent on the replication times of both the virus and its host. A bacteriophage plaque count can be used to determine the titer , the concentration of phage particles, in the suspension provided.