Viruses, Viroids, and Prions

Microbiology: Viruses, Viroids, and Prions

History of Virology

• 1892: Iwanowski worked with diseased tobacco plants.

  • Discovered tobacco mosaic virus was filterable through a filter that stops bacteria, indicating it was not influenced by dilution.

• Evolution of Viruses: The origins of viral evolution remain uncertain. Summary of hypotheses

General Characteristics of Viruses

• Definition: Obligatory intracellular parasites.

• Infection Range: Infect all forms of life.

• Genetic Material: Can contain DNA or RNA (single-stranded or double-stranded).

• Segmentation: Viral genomes can be segmented.

• Structural Components:

  • Protein Coat (Capsid): Comprised of capsomeres.

  • Envelopes: Some viruses have an additional envelope, and thus may also have spikes/peplomers that act as surface proteins.

• Host Range: Viruses typically infect only specific types of cells within one host (limited host range).

Virus Sizes

• Examples of virus sizes:

  • Adenovirus: 90 nm

  • Bacteriophage T4: 225 nm

  • Rabies virus: 170 x 70 nm

  • Poliovirus: 30 nm

  • Tobacco mosaic virus: 250 x 18 nm

  • Ebola virus: 970 nm

  • Human red blood cell: 10,000 nm in diameter

Virus Structure

• Virion Components:

  • Nucleic Acid: DNA or RNA

  • Capsid: Made of capsomeres.

  • Envelope: Surrounding the capsid in some viruses.

  • Spikes: For attachment to host cells.

Morphology of Viruses

• Helical Viruses: (e.g., Ebola virus)

• Complex Viruses: Some viruses exhibit irregular shapes.

• Enveloped Viruses: Include an outer lipid envelope that may help with infection.

Cultivation of Viruses

• Growth Requirements: Viruses can only be cultivated within living cells.

• Bacteriophages: Form plaques on bacterial lawn.

• Animal and Plant Viruses: Need to grow in live animals, embryonated eggs, or cultured cells.

Virus Identification

• Methods of Identification:

  • Serological Tests: Detect patient antibodies against viruses.

  • Nucleic Acid Tests: PCR (Polymerase Chain Reaction), RFLPs (Restriction Fragment Length Polymorphisms).

Bacteriophage Multiplication: The Lytic Cycle

1. Attachment: Phage attaches to host cell via tail fibers.

2. Penetration: Lysozyme opens cell wall; DNA injected into cell.

3. Biosynthesis: Production of viral components (DNA and proteins).

4. Maturation: Assembly of viral particles.

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

Lysogenic Cycle

• Process: Prophage DNA integrates into host bacterial DNA and can switch back to lytic cycle.

• Specialized Transduction: Involves acquisition of host DNA during phage replication.

Multiplication of Animal Viruses

1. Attachment: Viruses adhere to cell membrane.

2. Penetration: Through endocytosis or fusion.

3. Uncoating: Viral or host enzymes remove capsid.

4. Biosynthesis: Production of nucleic acids and proteins inside the host.

5. Maturation: Assembling viral components.

6. Release: Through budding or cell rupture.

Virus-induced Disease Mechanisms

• Cell Damage Mechanisms:

  • Lysis of host cells, immune-mediated cell damage, programmed cell death, and tumor formation.

Oncogenic Viruses

• Cancer Risk Increasers:

  • DNA Viruses: Include adenoviruses, herpesviruses.

  • RNA Viruses: Retroviruses (e.g., HIV) can insert DNA into host genome.

Taxonomy of Viruses

• Naming Convention: Family names end in -viridae, genus endings in -virus.

• Subspecies: Designated by a numeral.

• Example Families: Herpesviridae (e.g., Human herpes virus), Retroviridae (e.g., HIV).

Treatment and Prevention of Viral Infections

• General Treatments: Use nucleoside analogues and viral enzyme inhibitors.

• Nucleoside analogues are fake building blocks of DNA and RNA that stop viruses from multiplying. They are used in some antiviral medicines.

• Vaccines: Available for certain viruses like HPV and hepatitis.

Prions and Viroids

• Prions: Infectious proteins that cause neurodegenerative diseases such as Creutzfeldt-Jakob disease and mad cow disease.

• Viroids: Small, circular pieces of RNA that cause disease in plants; do not produce proteins and replicate independently.

Spikes, also referred to as peplomers, are surface proteins found on some viruses. Their primary functions include:

• Attachment to Host Cells: Spikes facilitate the initial attachment of the virus to the specific receptors on the host cell membrane, which is crucial for viral entry into the host cell.
• Facilitating Entry: Upon attachment, spikes can help mediate the fusion of the viral envelope with the host cell membrane, allowing the virus to penetrate the cell.

In summary, spikes play a critical role in the virus's ability to infect host cells through recognition and binding, thus initiating the infection process.