FUNDAMENTALS

I. VIRUSES

A. Definitions

  • Virus: A minute infectious agent, filterable and sub-microscopic, approximately 20-300 nm in size, that consists of either DNA or RNA wrapped in a protein coat.
  • Infectious Agent: An agent composed of nucleic acid (RNA or DNA) and a protein shell (capsid), and in some cases, a lipid envelope, capable of replication only in living cells using the host cell's machinery.
  • Virion: The complete virus particle, with full capacity for replication when a susceptible target cell is encountered.

B. Structure

  • Capsid: The protein coat surrounding viral nucleic acid, composed of repeating protein subunits called capsomeres. Capsids exhibit either helical or icosahedral symmetry.
  • Nucleocapsid: The complex of protein and nucleic acid.

C. Characteristics

  • Obligate Intracellular Parasites: Viruses cannot replicate outside living cells. They cannot generate energy or synthesize proteins independently.
  • Replication Process: One virus can replicate to produce hundreds of progeny viruses, differing from cellular division mechanisms like binary fission or mitosis.

D. Chemical Composition

  • Viruses consist of:
    • Nucleic Acid: 1-40%
    • Protein: 70-90%
    • Lipids: 15-35% (including phospholipids, glycolipids, cholesterol)

E. Size

  • Viral particles typically range from 20 to 300 nm in diameter, comparable to sizes of large proteins to small cells.

F. Shape

  • Basic shapes of viruses include:
    • Icosahedral
    • Helical
    • Complex (e.g., pox viruses)

G. Taxonomic Characteristics

  • Viral classification is less structured than other organisms and is based on features like nucleic acid type, strandedness, presence of an envelope, capsid shape, and host specificity.

H. Viral Constituents

  • Some viruses also include additional structures like an envelope derived from the host cell membrane, which may include spike-like proteins (glycoproteins).

II. ENVELOPED VIRUSES

A. Structure And Shape

  • Envelope: A lipid-containing membrane acquired during viral maturation. Not all viruses have envelopes.
  • Enveloped viruses can exhibit various shapes due to the fluid nature of the envelope.

B. Viral Envelopes

  • Envelopes provide:
    • Protection against the host immune system.
    • Facilitation of fusion with host cell membranes during infection.

C. Characteristics

  • Enveloped viruses are more sensitive to heat, detergents, and solvents than non-enveloped viruses, which tend to be more stable in adverse conditions.
  • Enveloped viruses require certain environmental conditions to maintain stability.

D. Functions Of Viral Capsid/Envelope

  • Protects viral nucleic acid from enzymatic degradation.
  • Facilitates introduction of viral genetic material into host cells.
  • Stimulates immune response, signaling production of antibodies.

III. NON-ENVELOPED VIRUSES

A. Virus Structure And Shape

  • Naked Viruses: Composed only of nucleic acid and capsid proteins. Typically more resistant to environmental conditions.

B. Taxonomic Characteristics

  • Different families of non-enveloped viruses (e.g., Picornaviridae) demonstrate varying pathogenicity and infectivity.

C. Viral Nucleic Acids

  • The viral genome can be linear or circular, and consist of either single-stranded or double-stranded RNA or DNA.

D. Viral Capsid And Symmetry

  • Capsid configurations include:
    • Icosahedral (20 triangular faces)
    • Helical (hollow coil)

E. Viral Proteins

1. Functions Of Surface Membrane Proteins
  • Viral surface proteins are crucial for attachment and entry into host cells, and also play a role in evading the host immune response.

IV. CLASSIFICATION OF VIRUSES

A. Properties Of DNA and RNA Viruses

DNA Viruses
  • Usually double-stranded, reside mainly in the nucleus, interact with host transcription machinery.
RNA Viruses
  • Generally single-stranded, replicate primarily in the cytoplasm, require RNA-dependent RNA polymerase for replication.

V. VIRAL PATHOGENESIS

A. Factors Involved In Viral Pathogenesis

  • Viral factors encoded in the genome influence the virulence and tropism of the virus.

B. Disease Caused By Viruses

  • Consequences of viral infections may include cell injury, tissue damage, and immune response variations leading to disease manifestations.
C. Sequential Steps In Viral Replication
  • Main Stages Include:
    1. Attachment
    2. Entry
    3. Uncoating
    4. Replication
    5. Maturation
    6. Release
D. Major Paths Of Virion Entry
  • Major routes of entry include respiratory tract, oropharynx, genital tract, and skin.

E. Fundamental Aspects Of Viral Pathogens

  • Viruses must bypass host defenses, spread effectively, and modulate host responses for successful replication and transmission.

F. Mechanisms Of Virus-Induced Disease

  • Pathogenesis can involve direct killing of host cells, immune-mediated damage, and chronic infection with potential for later disease complications.

G. Virus-Host Interactions

  • Interactions between viruses and host determine the severity and outcome of infections, including acute and chronic disease states.

H. Cellular Infection Types

I. Steps Of Pathogenesis
  • Virus entry, replication, spread, specific cellular infection, host immune interaction, and persistence may occur in various levels depending on the virus.

VI. ANTIVIRALS

A. Control Of Viral Infections

  • Antivirals target:
    • Viral replication enzymes (e.g., nucleoside inhibitors for reverse transcriptase)
    • Proteases inhibiting viral life cycles in infection.
    • Entry and exit pathways (e.g., inhibitors for HIV and influenza).

VII. REFERENCES

  • Use of antiviral agents, vaccines, historical context, and clinical practices were highlighted in academic resources such as Jawetz, Melnick, & Adelberg's Medical Microbiology.

VIII. PRACTICE QUESTIONS

  • Detailed comprehension questions focused on properties, classifications, and behaviors of viruses.

IX. APPENDIX

  • Includes additional insights, illustrations, and figures detailing various virus characteristics and examples for a clearer understanding of their operation.