The lysogenic cycle is a method of viral reproduction where a virus infects a host cell but does not immediately destroy it. This process is characteristic of temperate bacteriophages, viruses that infect bacterial cells. Instead of hijacking the cellular machinery for immediate particle assembly, the virus integrates its genetic material into the host’s genome. The host cell, known as a lysogen, continues to live and divide normally, passively carrying the viral DNA within its own chromosomes. This strategy allows the bacteriophage to persist and propagate across many generations of host cells without causing immediate harm.
The Stages of the Lysogenic Cycle
The cycle begins when a bacteriophage attaches to specific receptor sites on the surface of a bacterial cell, a process called adsorption. Following attachment, the virus injects its nucleic acid, typically double-stranded DNA, into the bacterial cytoplasm. The key event that distinguishes this cycle is integration, where the viral genome physically inserts itself into the bacterial chromosome with the assistance of an enzyme called integrase.
Once integrated, the viral DNA is termed a prophage, which is essentially a dormant state of the virus within the host genome. The prophage remains largely inactive, with only a few genes expressed to ensure its repression and dormancy. Every time the host bacterium divides, the integrated prophage DNA is replicated along with the host’s genetic material and passed on to daughter cells. This passive replication allows the viral genome to spread rapidly throughout a bacterial population without producing new viral particles or killing the host.
Contrasting the Lytic Pathway
The lysogenic cycle contrasts with the lytic pathway, the alternative life cycle available to temperate phages. In the lytic cycle, the phage genome takes control of the host cell’s machinery immediately after infection to produce new viral components. This takeover leads to the rapid synthesis, assembly, and release of hundreds of new viral progeny within a short timeframe.
The lytic cycle culminates in the lysis, or rupture, of the host cell membrane, which instantly destroys the bacterium and releases the newly formed viruses to infect other cells. Conversely, the lysogenic cycle is characterized by latency or dormancy, where the host cell survives and reproduces normally. The choice between these two pathways represents a survival strategy for the virus: immediate mass replication (lytic) or prolonged, quiet propagation (lysogenic).
What Triggers the Switch to Active Replication
The dormant prophage can transition into the destructive lytic cycle through a process known as induction. This switch is typically initiated by environmental stress factors that signal the host cell is in danger or dying. Such stressors can include exposure to ultraviolet (UV) radiation, treatment with certain chemicals, or conditions of nutrient deprivation.
The presence of DNA-damaging agents activates a repair mechanism in the bacterium, known as the SOS response, which involves the RecA protein. RecA initiates a cascade that results in the cleavage of a specific repressor protein that had been keeping the prophage genes silenced. Once the repressor is deactivated, the prophage DNA excises itself from the host chromosome (excision). The freed viral genome then expresses the genes necessary for the lytic pathway, leading to the rapid synthesis of new viral particles and the destruction of the host cell.
How Integrated Viruses Change Bacteria (Lysogenic Conversion)
Beyond mere replication, the presence of an integrated prophage can fundamentally change the characteristics of the host bacterium through a process called lysogenic conversion. This occurs when the viral genome carries genes that confer a new trait to the bacterium, altering its phenotype. The newly acquired genes can provide the bacterium with a selective advantage, often by encoding virulence factors that transform a harmless strain into a pathogen.
A significant number of human diseases are linked to bacteria that have acquired toxin genes via lysogenic conversion. This process is responsible for transforming several harmless bacterial strains into pathogens.
- Corynebacterium diphtheriae only produces the potent diphtheria toxin when lysogenized by a specific bacteriophage.
- Vibrio cholerae becomes capable of causing cholera after acquiring the gene for cholera toxin from a filamentous phage.
- The neurotoxin that causes botulism is carried by a prophage in Clostridium botulinum.
- Toxins that cause scarlet fever are found in Streptococcus pyogenes.