Bacteriophages Lecture Notes

Lytic vs. Lysogenic Cycle

  • Understanding the Cycles

    • Virulent Phages: These phages exclusively follow the lytic cycle, meaning they always induce lysis of the host cell upon infection.

    • Temperate Phages: These phages are capable of both the lytic and lysogenic cycles, allowing for coexistence with the host without immediate lysis.

  • Lytic Cycle Process

    1. Binding and Injection: The phage first attaches to a specific receptor on the bacterial surface, followed by the injection of its viral nucleic acid into the host cell.

    2. Early Gene Expression: This phase involves the activation of early promoters (PR and PL) and initial genome replication. Key proteins like the N protein act as an anti-terminator, extending transcription into genes encoding components for the phage head and tail.

    3. Late Gene Expression & Assembly: Structural proteins necessary for new virion assembly are produced.

    4. Inhibition of CI: The Cro repressor inhibits the CI protein, ensuring the progression towards the lytic pathway.

    5. Lysis: New virions are assembled within the host, and eventually, the host cell is lysed to release 100-1000 newly formed phage particles within approximately 30 minutes, contributing to a high phage titer (e.g., 101110^{11} plaque-forming units (pfu)/mL).

  • Lysogenic Cycle Decision and Process

    • Decision Factors: The choice between the lytic and lysogenic pathway is influenced by various factors, including nutrient availability in the environment and the multiplicity of infection (MOI).

    • Pivotal Proteins: The interaction and balance between pivotal proteins, particularly CII and Cro, determine which cycle proceeds.

    • Integration: In the lysogenic cycle, the viral genome, known as a prophage, integrates into the bacterial chromosome.

    • Lysogen: A bacterial cell harboring a prophage is called a lysogen, where the phage genetic material is replicated along with the host chromosome without causing immediate harm.

    • Lysogenic Conversion: The presence of a prophage can confer new properties to the bacterium, a process known as lysogenic conversion (e.g., encoding virulence factors that cause pathogenic traits).

    • Induction: The prophage can be induced to enter the lytic cycle, for instance, by environmental stressors or antibiotic exposure, leading to the expression of virulence factors or phage replication and subsequent lysis.