Chapter 6; Viruses Overview HARD

Acellular Pathogens: Viruses

• Definition: Viruses are submicroscopic, infectious, parasitic, and non-living agents. They consist of nucleic acid (DNA or RNA, which can be double-stranded, single-stranded, circular, or linear) surrounded by a protein coat (capsid).
• Capsids: Viruses have three structural forms for their capsid: helical, icosahedral, or complex (a combination of the first two).
• Host Range: This refers to the range of organisms a virus can infect. Tissue specificity within a host is also important, as different viruses can target specific tissues or organs (e.g., nervous system). Examples include smallpox (humans) and bacteriophages (certain bacterial species).
• Classification Criteria: Viruses are classified based on factors including the host organism, morphology (capsid structure), and genome type.
• +ssRNA vs. –ssRNA: +ssRNA directly acts as a messenger for protein synthesis. –ssRNA requires a template to be converted to +ssRNA for protein synthesis.

Bacteriophages

• Definition: Bacteriophages are viruses that infect and replicate within bacterial cells.
• Replication Steps: Bacteriophage replication proceeds through five phases:
  • Attachment: The phage attaches to the host bacterium.
  • Penetration: Viral DNA enters the host cell.
  • Biosynthesis: The phage's DNA replicates, and its proteins are produced.
  • Maturation: New phage particles are assembled.
  • Lysis: The host cell is broken open (lysed), releasing newly assembled phages. The capsid does not enter the cytoplasm during this process.
• Virulent vs. Temperate Phages:
  • Virulent Phages: Undergo a lytic cycle, resulting in the cell's death.
  • Temperate Phages: Undergo a lysogenic cycle, where they incorporate their DNA into the host chromosome and replicate along with the host cell. These remain latent until activation (i.e., stress). Stress can trigger the switch to a lytic cycle.

Animal Virus Replication

• Differences from Bacteriophages: Animal virus replication differs from bacteriophage replication significantly. Primarily in that there the entire virus, not just the nucleic acid, may enter the cytoplasm.
• Uncoating: This is the process by which the viral nucleic acid is released from the protein coat within the host cell.
• Penetration Mechanisms:
  • Enveloped Viruses: Fuse with the host cell's plasma membrane to release their nucleocapsid.
  • Non-enveloped Viruses: Are taken in by endocytosis into the host cell.
• +ssRNA and –ssRNA Replication (Animal Viruses):
  • +ssRNA: Directly used by the host cell's ribosomes to synthesize proteins.
  • –ssRNA: Requires creation of the complementary +ssRNA as a template before protein synthesis is possible.
• Release Mechanisms:
  • Enveloped: Budding from the host cell membrane, or direct extrusion.
  • Non-enveloped: Lysis of the host cell.

Retroviruses & Others

• Retroviruses: RNA viruses that use reverse transcriptase to produce DNA from their RNA.
• Viral Growth Curve: The process of viral replication in a host cell that stages as inoculation, eclipse, and burst (release into the medium). Burst size is the maximum number of virions produced per bacterium.
• Animal Virus Cultivation: Viruses can be cultivated in:
  • Fertilized Chicken Eggs:
    • Amniotic cavity
    • Chorioallantoic membrane
    • Yolk sac
  • Cell Cultures:
    • Primary: Isolated from animal tissues, have a limited lifespan and limited number of divisions. Affected by contact inhibition. The ability to grow stops as the cells begin to touch one another.
    • Secondary: Developed from primary cell cultures, grow longer than primary, but are still limited.
    • Continuous: Derived from transformed cells (tumors), can be subcultured indefinitely without limitations. Not affected by contact inhibition.
• Bacteriophage Cultivation: Grown on a bacterial lawn (dense layer of bacterial cells).
• Plaques: Clear zones in a bacterial lawn, indicating areas where the bacteria have been killed by the phage.
• Virus Detection: By looking for changes in the host cell (cytopathic effects), for example:
  • Cytopathic Effects (CPEs):
    • Syncytia: Fusion of host cells into multinucleated giant cells.
    • Cell inclusions: Accumulations of viral proteins or substances forming structures within the cell.
• Virioids: Non-infectious, single-stranded RNA molecules that cause plant diseases. They need a host to replicate but do not encode proteins to do it themselves.
• Virusoids: Infectious single-stranded RNA molecules needing a helper virus for replication. They do not encode proteins.
• Prions: Infectious, self-replicating proteins causing diseases. They are misfolded forms of normal cellular proteins (PrP^C and PrP^Sc). PrP^Sc converts normal PrP^C into the pathogenic form, causing the diseases by accumulating in the CNS. Prion diseases are spread through the conversion process.
• Antiviral Drugs: Some drugs block viral steps, for example: penetration/uncoating, genome replication, or virion maturation/release. Examples of antiviral drugs are given.