chapter 13

Chapter 13: Viruses, Viroids, and Prions

Key Terms

  • Bacteriophage: A virus that infects bacteria, commonly referred to as a phage.

  • Latent Infection: A viral infection where the viral genome persists silently within the host cell without producing new viral particles.

  • Lysogen: A bacterium harboring phage DNA integrated into its genomic material.

  • Lysogenic Conversion: Alteration of one or more bacterial properties upon acquiring a prophage.

  • Lytic Infection: A productive viral infection resulting in the host cell's lysis (bursting).

  • Prion: An infective protein associated with neurodegenerative diseases.

  • Productive Infection: A viral infection producing multiple new viral particles.

  • Viroid: An infectious RNA agent that affects plants, consisting solely of RNA.

  • Virion: The complete viral particle in its inactive form, outside a host cell.

Historical Context

  • Tobacco Mosaic Disease:

    • In the 1890s, identified as caused by a “filterable virus,” too small to visualize with a light microscope.

    • The discovery by D. M. Iwanowsky and Martinus Beijerinck led to the understanding of viruses as infectious agents different from bacteria.

    • F. W. Twort and F. d’Herelle identified phages that could destroy bacteria.

    • The term “virus” is derived from the Latin for “poison.”

Introduction to Viruses

  • Characteristics:

    • Viruses are composed of genetic material (DNA or RNA) encased in a protective protein coat but are inert outside hosts.

    • They lack metabolic activity, replication capabilities, and movement and require living hosts for propagation.

    • Cannot be cultured in pure media and cannot be observed using light microscopy.

    • Classifications are generally based on whether they infect eukaryotic or prokaryotic cells.

Obligate Intracellular Parasites

  • Bacteriophages (Phages):

    • Viruses that specifically infect bacteria, instrumental in laboratory studies to understand virus-host interactions.

    • Serve as vectors for gene transfer and play crucial ecological and medical roles.

Relative Sizes of Viruses

  • Virus sizes vary from 30 nm (Poliovirus) to 800 nm (Mimivirus).

  • Relative sizes in comparison to other microorganisms includes:

    1. Poliovirus (30 nm)

    2. Hepadnavirus (42 nm)

    3. Adenovirus (90 nm)

    4. T4 Bacteriophage (225 nm)

    5. Tobacco Mosaic Virus (250 nm)

    6. E. coli (3,000 x 1,000 nm)

    7. Human RBC (10,000 nm)

Viral Structure

  • Components:

    • Virion consists of nucleic acids (DNA or RNA) encased in a capsid formed from identical protein subunits called capsomeres.

    • Enveloped viruses have an additional lipid bilayer derived from host membranes, while non-enveloped viruses are more resistant to disinfectants.

    • Shapes of viruses include icosahedral, helical, and complex forms like bacteriophages.

Transmission Routes of Human Viruses

  • Groups and Mechanisms:

    • Enteric: Fecal-oral transmission (e.g., Enteroviruses).

    • Respiratory: Salivary and respiratory droplets (e.g., Influenza).

    • Sexually Transmitted: Via sexual contact (e.g., HIV).

    • Zoonotic: From animals to humans (e.g., Rabies).

Bacteriophages

Types

  • Lytic Phages: Cause the immediate death of host cells by lysis.

  • Temperate Phages: Have the option to enter a latent phase, integrating their genome into the host genome without causing immediate cell death.

  • Filamentous Phages: Cause productive infections without killing the host immediately.

Infection Cycle of Lytic Phages

  1. Attachment: Phage binds to receptors on the host cell.

  2. Genome Entry: Phage lysozyme degrades the cell wall, allowing DNA entry.

  3. Synthesis: Host's machinery is hijacked to replicate phage DNA and produce proteins.

  4. Assembly: New viral particles are assembled.

  5. Release: Host cell lyses, releasing new virions.

Lysogenic Cycle

  • Integration: Phage DNA integrates into host genome, becoming a prophage.

  • Excision: Prophage can be induced to exit and enter the lytic cycle, particularly in response to environmental stresses.

  • Consequences: Lysogenic conversion can lead to new traits in bacteria, such as virulence factors.

Microbial Pathogen Examples of Lysogenic Conversion

  • Clostridium: Produces botulinum toxin.

  • Corynebacterium: Produces diphtheria toxin.

  • Escherichia coli O157:H7: Produces Shiga toxin.

Filamentous Phages (e.g., M13)

  • Cause productive infections, allowing slow release of new virions without killing the host cell.

  • Entry Mechanism: Attach to E. coli via the F pilus; uses host machinery to replicate.

Horizontal Gene Transfer by Bacteriophages

  • Generalized Transduction: Random bacterial DNA is packaged into a phage during assembly.

  • Specialized Transduction: Specific bacterial genes next to the prophage DNA are packaged when a lysogenic phage enters the lytic lifecycle.

Studying Bacteriophages

  • Plaque Assays: Used to quantify phages by measuring zones of bacterial lysis on an agar plate.

Animal Virus Replication Cycle

  • Stages:

    1. Attachment (adsorption)

    2. Entry and uncoating

    3. Synthesis of viral components

    4. Assembly of new virions

    5. Release from host cell.

Types of Viral Infections in Animals

  • Acute: Rapid onset, short duration (e.g. Influenza).

  • Persistent: Long-term infections that may or may not cause symptoms. Can be chronic or latent.

Cancers and Viruses

  • Oncoviruses: Certain viruses can cause cancers by integrating into host DNA and altering growth regulatory pathways.

  • Examples: Epstein-Barr virus, Hepatitis B virus, Human Papillomavirus.

Cultivation of Viruses

  • Cultivated in cell cultures or fertilized eggs. Morphological changes indicate viral infections.

Plant Viruses

  • Cause economic damage; symptoms include stunted growth and tissue malformation.

  • Transmission occurs through vectors and through physical entry from wounds.

Viroids and Prions

  • Viroids: Small RNA molecules causing disease in plants.

  • Prions: Infectious proteins causing neurodegenerative diseases (e.g. Creutzfeldt-Jakob disease), resistant to traditional sterilization methods.