Microbiology charter 6

Chapter Overview

  • Chapter 6: An Introduction to Viruses, Viroids, and Prions from Talaro’s Foundations in Microbiology

  • Importance of understanding viruses as they change various aspects of biological life.

The Position of Viruses in the Biological Spectrum

  • Origin of Viruses: No universal consensus on how and when viruses originated.

  • Abundance: Viruses are the most abundant microbes on Earth.

  • Nature: They are obligate intracellular parasites, requiring a host to replicate.

Size of Viruses

  • Ultramicroscopic: Most viruses are less than 0.2 μm in size, needing electron microscopy for visualization.

Viral Structure

  • Lack of Cell Complexity: Viruses do not resemble cells and lack the machinery for protein synthesis.

  • Components:

    • Covering: Capsid (protein coat) and possibly an envelope.

    • Central Core: Contains nucleic acid (DNA or RNA) and may include matrix proteins and enzymes.

Capsids

  • Definition: Protein coats that protect the viral nucleic acid, forming the nucleocapsid.

  • Types:

    • Helical: Long, cylindrical structures.

    • Icosahedral: Symmetrical 3D shapes with 20 sides and 12 corners.

Viral Envelope

  • Presence: Mostly found in animal viruses; acquired during the viral release from the host cell.

  • Function of Spikes: Exposed proteins essential for attaching to host cells.

Functions of Capsid and Envelope

  • Protects nucleic acid outside host cells.

  • Aids in binding and penetration of the host cell to transfer viral DNA or RNA.

Complex Viruses

  • Attraction of Atypical Structures: Some have unique structures (e.g., Poxviruses lack a typical capsid).

  • Bacteriophages: Complex viruses that infect bacteria, with unique structures including a polyhedral nucleocapsid and tail fibers.

Viral Classification

  • Based on structure, chemical composition, and genetic material.

  • Taxonomy: 59 orders, 189 families, and 2,224 genera recognized.

    • Family names end in -viridae, genera end in -virus.

Important Human Virus Families: DNA Viruses

  • Poxviridae: Variola causes smallpox.

  • Herpesviridae: Includes HSV-1, HSV-2, etc. Causes various herpes infections and chickenpox.

  • Adenoviridae: Causes adenovirus infections.

  • Papillomaviridae: Includes Human Papillomavirus (HPV), causing warts.

Important Human Virus Families: RNA Viruses

  • Picornaviridae: Includes poliovirus and hepatitis A.

  • Caliciviridae: Norwalk virus, which causes viral diarrhea.

Viral Multiplication Cycle in Animals

  • Phase Overview:

      1. Attachment: Virus binds to host cell.

      1. Penetration: Nucleocapsid enters the host cell.

      1. Uncoating: Viral nucleic acid is released.

      1. Synthesis: New viral components produced.

      1. Assembly: New virus particles are constructed.

      1. Release: Viruses exit the cell (budding or lysis).

Damage to Host Cells

  • Cytopathic Effects: Observable damage includes alterations in cell shape, size, and introduction of inclusion bodies.

  • Persistent infections may lead to chronic conditions or reactivation of dormant viruses (e.g., herpes viruses).

Oncogenic Viruses

  • Certain viruses can cause cancer (oncoviruses) by integrating genetic material into host DNA, leading to cellular transformation and tumor formation.

    • Examples: HPV (cervical cancer) and Epstein-Barr virus (Burkitt’s lymphoma).

Bacteriophage Multiplication Cycle

  • Bacteriophages: Virus that targets bacteria, using E. coli as a model. Uncoating is not required; viruses are released through cell lysis.

Cultivating and Identifying Animal Viruses

  • Goals: Isolate and identify viruses, prepare for vaccines, and conduct research.

  • Methods:

    • In vitro: Cell cultures for replication.

    • In vivo: Use of bird embryos and live animals for study.

Medical Importance and Detection of Viruses

  • Epidemiology: High incidence of acute viral infections.

  • Detection Methods: Include culture, antibody presence, or direct observation of cytopathic effects (CPE).

Prions and Nonviral Infectious Particles

  • Prions: Infectious proteins causing fatal neurodegenerative diseases, resistant to sterilization.

  • Characteristics: No nucleic acid, responsible for diseases such as CJS and Kuru.

  • Other agents: Satellite viruses depend on other viruses; viroids consist of RNA with no protein coat, affecting plants.