Lecture 10/10

Introduction to Viruses

  • Focus on bacteriophages (phages) as the first type of viruses studied.

    • Phages exclusively infect bacteria.

    • Understanding phage replication provides insight into complex viruses that infect human cells.

    • Bacteriophages possess significant potential for treating bacterial infections.

Replication of Bacteriophages

  • All viruses require a host cell for replication; they cannot replicate independently.

  • Replication involves several key steps common to all viruses, starting with the attachment to host cells.

Steps in Bacteriophage Replication

  1. Attachment (Adsorption)

    • Definition: Phages attach to specific molecules on the surface of bacterial cells (receptors).

    • Importance: Attachment is a critical first step for viral infection and replication.

    • Receptors vary and may include:

      • Lipopolysaccharides on gram-negative bacteria.

      • Quorins, such as ONF, present in bacterial membranes.

      • Transporter proteins that span the cell envelope.

      • Flagellar components or pili.

    • Receptors are normal bacterial molecules and are not produced specifically to attract phages.

  2. Penetration

    • Definition: The process through which phages inject their DNA into the host cell.

    • Mechanism: The phage uses a hollow protein structure to pierce the bacterial cell envelope, allowing the DNA to be injected while leaving the protein shell outside.

    • Result: Only the phage's DNA enters the bacterial cytoplasm, initiating a takeover.

  3. Synthesis

    • Definition: The bacterial host synthesizes new phage DNA and proteins based on the instructions of the injected phage DNA.

    • Process:

      • The phage uses the host's cellular machinery, including ATP and ribosomes, to produce viral components.

      • The host’s DNA is often destroyed to access nucleotides needed for new viral DNA synthesis.

    • Implication: The bacterial cell is transformed into a "phage factory."

  4. Assembly

    • Definition: Newly synthesized phage parts (DNA and proteins) are assembled into complete viral particles.

    • Mechanism: All components self-assemble into newly formed phages, preparing for release.

  5. Lysis (Release)

    • Definition: The bacterial cell bursts, releasing new phage particles into the environment.

    • Mechanism: Bacteriophages produce their own lysozyme which breaks down the bacterial cell wall from within, allowing lysis.

    • Result: A single phage infection can lead to the production of tens to hundreds of new phages (known as burst size). Example: Some phages produce an average of 93 new phages per infection.

    • Timeframe: Total replication cycle can take about 30 to 45 minutes.

Visualization of Phage Replication

  • Electron Microscopy:

    • Initial observed state: Bacterial cells (like E. coli) can be seen covered with phages.

    • Post-replication state: Cells show significant lysis with visible release of phage particles.

  • Plaques Formation:

    • In a culture plate, areas of clearing (plaques) appear where bacteria have been lysed by phages, indicating phage replication and spread.

  • Turbidity Change:

    • The turbidity of a culture diminishes as the phage infects and lyses bacterial cells, leading to a clear medium where cells once proliferated.

Lytic Cycle Overview

  • Definition: The process where phage infection leads to the death of the bacterial host (lysis).

  • Example: The T4 phage undergoing the lytic cycle effectively demonstrates this process.

  • Note: Viruses are often referred to as "bad news wrapped in protein" due to their destructive nature when infecting and replicating within host cells.

Lysogenic Cycle

  • Temperate Phages

    • Definition: Some bacteriophages can integrate their DNA into the host chromosome without immediately killing the bacterial cell.

    • Process:

      • Phage DNA integrates into the bacterial chromosome and replicates along with it without killing the host.

    • Implication: This can potentially confer new genetic traits or functions to the bacteria, providing further benefits or adaptations.

    • Transition: The integrated phage DNA can later be induced to exit the bacterial chromosome and enter the lytic cycle under certain conditions, leading to cell lysis and phage production.

Summary

  • Phages have diverse mechanisms for interacting with bacterial hosts, primarily through distinct cycles of replication (lytic and lysogenic).

  • Understanding phage biology has critical implications for fields such as biotechnology, medicine, and microbiology, particularly in developing phage therapy against antibiotic-resistant bacteria.

  • Further research into lysogenic cycles and their potential impacts on host bacteria can lead to new applications in genetic engineering and therapeutic strategies.