Lysogeny is a stable association between a temperate bacteriophage and its bacterial host in which the phage genome integrates into the host chromosome or persists as a plasmid, forming a prophage. During lysogeny, the phage does not cause immediate lysis but replicates passively as the host cell divides.
Mechanism and Regulation
Temperate bacteriophages enter lysogeny after infecting a susceptible bacterium when conditions favor a latent state over immediate replication. Following injection of the phage DNA, integrase enzymes mediate recombination between phage and host sequences, inserting the viral genome into a specific site in the chromosome. In some cases the genome remains extrachromosomal as a plasmid. A phage-encoded repressor protein maintains lysogeny by blocking transcription of lytic genes and conferring immunity to superinfection by the same or related phages. The prophage replicates synchronously with the host genome, ensuring its stable inheritance. Environmental stresses such as UV radiation, certain antibiotics, or nutrient deprivation can inactivate the repressor and trigger induction. Once induced, the prophage excises from the genome and enters the lytic cycle, producing new virions and lysing the host. Lysogeny can influence bacterial physiology and evolution by providing new genes, including virulence factors and metabolic enzymes, through lysogenic conversion. For example, phage-encoded toxins contribute to pathogenicity in Corynebacterium diphtheriae and Vibrio cholerae.
Notable Examples and Effects
Lambda phage is the classical model of lysogeny; it integrates into the Escherichia coli chromosome via the attB site and is maintained by the cI repressor. Under stress, lambda prophage induction results in a burst of phage particles and host cell lysis. In Streptococcus pyogenes, the prophage encoding streptococcal pyrogenic exotoxins increases virulence and is responsible for scarlet fever. The Shiga toxin genes in certain strains of Escherichia coli and the cholera toxin genes in Vibrio cholerae are carried on lysogenic phages and are essential for disease. Lysogeny also plays a role in horizontal gene transfer, spreading antibiotic resistance and metabolic traits among bacterial populations. Not all lysogenic relationships are detrimental; some prophages encode factors that protect the host from oxidative stress or confer competitive advantages in specific environments.
Lysogeny allows bacteriophages to persist within bacterial hosts without destroying them, balancing viral propagation with host survival. This intimate relationship shapes microbial ecology and contributes to the evolution of bacterial pathogens through the acquisition of phage‑encoded genes.
Related Terms: Temperate Phage, Prophage, Lytic Cycle, Bacteriophage, Lysogenic Conversion