Lytic and lysogenic cycles Knowledge-Based Study

Study notes

1. Overview of Bacteriophage Reproduction

   Viruses are obligate intracellular parasites that lack metabolic machinery, ribosomes, and the enzymes necessary for independent protein synthesis and energy production. Consequently, they depend entirely on a permissive host cell to replicate. Viruses that specifically infect bacterial hosts are known as bacteriophages (or simply phages).

   Bacteriophages, such as the well-studied T-even phages and phage lambda ($\lambda$), interact with bacterial cells through two principal reproductive pathways:

   • The Lytic Cycle: A virulent pathway resulting in the rapid destruction and lysis of the host bacterium.
   • The Lysogenic Cycle: A temperate pathway in which the viral genome integrates stably into the bacterial chromosome, persisting without immediate destruction of the host.

   Understanding these distinct pathways illustrates the dynamic balance between immediate viral propagation and latent persistence within biological populations.

   Stages of the Lytic Pathway

   In the lytic cycle, a virulent bacteriophage commandeers the host's cellular machinery to synthesize new virions, culminating in the rupture of the host cell wall. This cycle proceeds through five sequential stages:

   1. Attachment (Adsorption): Specific tail fibers of the bacteriophage bind to complementary receptor proteins or lipopolysaccharides on the bacterial cell wall surface.
   2. Penetration (Entry): The phage tail sheath contracts, driving a hollow core tube through the bacterial cell wall and plasma membrane. The viral genome (typically double-stranded DNA) is injected into the cytoplasm, leaving the empty protein capsid outside as a "ghost".
   3. Biosynthesis: Viral enzymes degrade the host bacterial chromosome. The host cell's RNA polymerases and ribosomes are redirected to transcribe and translate viral genes, producing viral structural proteins, replication enzymes, and copies of the viral genome.
   4. Maturation (Assembly): Newly synthesized viral components assemble spontaneously into complete, infectious virions. Capsid heads are packaged with viral DNA, and tail assemblies are attached.
   5. Lysis and Release: The phage synthesizes late proteins such as endolysin (which degrades peptidoglycan) and holin (which perforates the inner membrane). Weakened by internal osmotic pressure and enzymatically digested cell wall structure, the bacterium bursts, releasing hundreds of progeny phages into the surrounding environment to infect adjacent cells.

   The Temperate Lifecycle and Prophage State

   Unlike virulent phages, temperate phages (such as bacteriophage lambda) have the capacity to enter the lysogenic cycle. This cycle is characterized by coexistence between the virus and host without immediate cell lysis.

   The lysogenic cycle involves the following key events:

   • Attachment and Entry: Just as in the lytic cycle, the phage attaches to specific surface receptors and injects its viral DNA into the host bacterium.
   • Integration: Instead of destroying host DNA, viral integrase facilitates the site-specific recombination of the viral genome into the host's circular bacterial chromosome. Once integrated, the dormant viral genome is referred to as a prophage.
   • Repression: Specialized repressor proteins encoded by the viral genome block the transcription of lytic genes, preventing premature entry into the destructive lytic pathway.
   • Lysogenic Replication: Every time the host bacterium replicates its own chromosome via binary fission, it replicates the integrated prophage alongside it. The viral genome is thus inherited by all daughter cells indefinitely without harming the host.

   A bacterium harboring a prophage is designated a lysogen (or lysogenic bacterium). In many cases, prophages confer novel phenotypic traits to their hosts through a phenomenon known as lysogenic conversion (e.g., granting toxin-producing capabilities to non-pathogenic bacteria).

   Induction: Transition from Lysogeny to Lysis

   The lysogenic state is not necessarily permanent. When a lysogenic bacterium experiences environmental stressors—such as ultraviolet (UV) radiation, chemical mutagens, or nutrient starvation—the host SOS repair response is triggered. This cellular stress inactivates the viral repressor protein.

   As repression fails, the prophage undergoes induction: the viral DNA is excised from the host chromosome and immediately enters the lytic cycle, initiating biosynthesis, assembly, and subsequent host cell lysis.

   Key Differences Between Cycles

   • Fate of Host: The lytic cycle invariably kills the host cell via lysis, whereas the lysogenic cycle allows host survival and vertical transmission of viral DNA.
   • Viral State: The lytic cycle produces free infectious virions; the lysogenic cycle maintains the viral genome as an integrated, non-infectious prophage.
   • Phage Types: Virulent phages replicate exclusively via the lytic cycle, whereas temperate phages are capable of alternating between both cycles.
   • Duration: The lytic cycle is rapid (typically minutes to hours), whereas the lysogenic cycle can persist across hundreds of bacterial generations.