Viruses

Introduction to Microbiology and Viruses

What do you already know?

• Key Questions:

  • Differences between viruses and bacteria?

  • Are all bacteria harmful?

  • Should antibiotics be taken for a cold?

  • Best practices to avoid getting sick?

Microbiology

• Definition: The study of microscopic organisms including:

  • Viruses

  • Bacteria (Archaea and Eubacteria)

  • Protists (e.g., paramecium)

  • Some Fungi (e.g., yeast)

Viruses

• Characteristics:

  • Non-cellular particles

  • Composed of genetic material (either DNA or RNA) and protein

  • Ability to invade all types of life forms: animals, plants, and microorganisms (e.g., eubacteria, archaea)

• Examples of viruses: HIV, Herpes, Influenza

Structure of Viruses

• Lack cellular structures like:

  • Cell membrane

  • Membrane-bound organelles (nucleus, mitochondria, Golgi apparatus)

• Inactivity outside a host cell

  • Viruses are entirely inert until they invade a host cell

Components of Viruses

1. Capsid

   • Protein coat providing protection for genetic material

2. Genetic Material (DNA/RNA)

   • Instructions for replication within the host cell

3. Enzymes

   • Facilitate viral entry and takeover of the host cell

4. Specific Envelope Proteins

   • Allow virus attachment to specific host cells, e.g., bacteriophages have tail fibers for this purpose

Viral Life Cycle

Lytic Cycle

• Definition: Involves takeover of the host cell to produce new viruses.

Steps:

1. Attachment: Virus connects to host cell surface.

2. Entry: Virus injects genetic material, using an enzyme to weaken entry point.

3. Replication & Assembly: Viral genetic information commandeers host machinery to create virus components.

4. Lysis: Host cell bursts, releasing new viruses and continuing the cycle.

5. Virulence: Indicates the infectious capacity of a virus based on its ability to complete the lytic cycle.

Lysogenic Cycle

• Definition: Host cell remains unaltered, viral DNA integrates into host DNA (forming a Prophage).

Steps:

1. Integration: Viral genetic material merges into the host DNA, generally non-destructive.

2. Cell Multiplication: Host cell divides normally, copying viral DNA alongside.

3. Latency: Prophage remains inactive while the host cell continues to function.

4. Activation: Triggers (like stress) may lead prophage to exit and switch to lytic cycle.

Viruses: Living or Non-Living?

• Characteristics of Life:

  • Growth: NO for viruses, YES for cells.

  • Homeostasis: NO for viruses, YES for cells

  • Metabolism: NO for viruses, YES for cells.

  • Genetic Material: BOTH have DNA/RNA

  • Reproduction: Only with a host for viruses, Cell division for cells

• Conclusion: Viruses are considered NON-LIVING.

Viruses and Diseases

• Examples of viral diseases include:

  • Herpes simplex

  • Chicken pox

  • HIV

  • Hepatitis

• Viral Specificity: Certain viruses infect specific hosts only.

The Body’s Defense Against Viruses

Primary Defense

• Prevents virus entry:

  • Skin (barrier unless breached)

  • Mucous membranes in respiratory tract (trap pathogens)

  • Hydrochloric acid in stomach (destroys viruses)

Secondary Defense

• Immune response:

  • White blood cells (WBCs) produced (lymphocytes, phagocytes)

  • Phagocytes engulf and destroy intruders.

Tertiary Defense (Antibody-Mediated Immunity)

• Produced by lymphocytes: Antibodies target specific pathogens.

• Interferons: Proteins from infected cells signaling nearby cells to prepare an immune response.

Oncogenic Viruses

• Definition: Viruses that can cause cancer.

• Mechanism: Insert DNA into host cell and can disrupt cell cycle regulation.

• Tumor Types:

  • Benign: Dormant, non-invasive cells.

  • Malignant: Active, invasive cancer cells.

Gene Therapy

• Innovative method potentially curing genetic diseases by delivering healthy genes into patient cells using viruses.

Process:

1. Alter virus to eliminate harmful effects.

2. Insert healthy gene into viral vector.

3. Virus introduces new gene into patient's cells, promoting production of necessary proteins.

4. Example: Cystic Fibrosis, leading to blockage in lungs, intestines due to thick mucus.

Diagram of Gene Therapy Process

• Steps include:

  • Virus binding to cell and releasing new gene into nucleus for gene expression

  • Ensuring altered cells can produce desired proteins and are injected back into the patient's body.