Modes of replication

Bacteriophages can have one of two reproductive strategies. In the first lytic mode of replication, the bacteriophage enters a suitable cell, replicates, and then kills its host. The alternative is a lysogenic mode. A whole virion infects the cell, but rather than triggering more virion production, the viral genome recombines with the host genome and does no further damage. The lysogenized virus’s only function is to code for proteins that prevent reinfection with bacteriophages of the same type.  The viral genome is passed on to daughter cells during normal host cell division. In response to an environmental stimulus the viral DNA excises from the host genome and a lytic cycle ensues. Virulent bacteriophages, such as T4, which only have a lytic replication cycle, run the risk of exterminating the host population. In the laboratory, these bacteriophages are characterized by the complete lysis of all the host cells in a liquid culture. Those with the ability to perform lysogeny can maintain a pool of host cells capable of releasing virions gradually. In liquid culture, Bacteria infected with lysogenic phages fail to reach the expected optical density, but still appear to be dividing normally.