lysogen

In microbiology, a lysogen is a bacterium that harbors a temperate bacteriophage genome integrated into its chromosome (as a prophage) or maintained as a plasmid. During the lysogenic cycle, phage DNA integrates via site-specific recombination facilitated by phage integrase, then replicates with the host. The prophage can be stably inherited as the cell divides and may later be induced to enter the lytic cycle.

Lysogeny confers immunity to superinfection by related phages through repressor proteins that maintain the lysogenic state.

Prophages can cause lysogenic conversion, in which phage genes alter host traits, including toxins or virulence factors. Classic examples include the diphtheria toxin gene carried by a β phage in Corynebacterium diphtheriae, cholera toxin genes carried by the CTX phage in Vibrio cholerae, and Shiga toxin genes in certain Escherichia coli strains.

Under stress such as DNA damage, the SOS response can induce prophage excision and the lytic cycle, leading to virion production and cell lysis.

Not all lysogens produce active phage; many prophages are defective and cannot form infectious particles. Nevertheless, the lysogenic state influences bacterial evolution by enabling horizontal transfer of phage genes and traits.