Recording-2025-03-16T20:30:07.628Z Lecture 13.2

Comparison of Viruses and Cells

• Acellular Nature of Viruses

  • Viruses are acellular (not composed of cells) but must be intracellular (within a cell) to replicate.

  • To enter a host cell, viruses must be smaller than the host cell.

• Size Comparison

  • E. Coli: Bacterial cell size is larger than viruses.

  • Red Blood Cells: Eukaryotic cells are small in size compared to bacteria and viruses.

    • Smallest cells in the human body.

    • Bacteria are significantly smaller than red blood cells.

    • Viruses are even smaller than bacteria; some viruses are comparable in size to ribosomes in bacterial cells.

  • Limit of Light Microscopes: Light microscopes can resolve up to 0.2 microns (200 nanometers), making it difficult to visualize viruses.

  • Electron Microscopy: To visualize small entities like viruses, electron microscopy is utilized.

Bacteriophage Overview

• Definition: Bacteriophages (phages) are viruses that infect bacteria.

  • Often referred to as 'bacteria eaters.'

• Visual Representation

  • Images show bacteriophages attached to bacterial surfaces.

  • Colors in scans are added for visualization purposes, not accurate to real color.

• Bacteriophage Structure

  • Composed of a capsid (head) that holds viral DNA, with tail fibers (legs) capable of puncturing bacterial cell walls.

Bacteriophage Life Cycle

• Lytic Cycle

  • Attachment: Phage attaches to the host bacteria using tail fibers.

  • Penetration: Punctures the cell wall and injects DNA into the bacterial cytoplasm.

  • Host Cell Takeover: Viral DNA directs the bacterial machinery to produce more viral components, exploiting the resources of the host.

  • Assembly: New bacteriophage parts are assembled inside the bacterial cell.

  • Lysis: The bacterial cell bursts, releasing new phages to infect further bacteria.

  • Result of Lytic Cycle: Areas of clearance on Petri dishes where bacteria have been destroyed by phages.

Lysogenic Cycle

• Comparison with Lytic Cycle

  • Some phages can enter the lysogenic cycle instead of immediately destroying the host cell.

• Integration: Viral DNA is integrated into the bacterial chromosome, replicating passively during bacterial cell division.

• Benefits to Host: The integrated phage DNA can provide beneficial genes, aiding bacterial survival and sometimes enabling pathogenicity (e.g. diphtheria requires certain phage genes to cause disease).

• Lysogenic to Lytic Switch: The phage may revert to a lytic cycle if the bacterial host experiences stress or unhealthy conditions, leading to a massive release of new phages.

Phage Therapy Potential

• Therapeutic Use: Research has explored the use of bacteriophages as a treatment for bacterial infections.

• Advantages Over Antibiotics

  • Phages are specific to particular bacterial strains, potentially reducing collateral damage to beneficial microbiota.

  • Phages can evolve alongside bacteria, potentially overcoming antibiotic resistance.

• Concerns with Antibiotic Use: Broad-spectrum antibiotics may harm beneficial bacteria, which phage therapy could avoid while specifically targeting harmful bacteria.

• Future Directions: Increased interest in phage therapy is emerging due to rising antibiotic resistance, representing a potential sustainable alternative for treating bacterial infections.