Applied Microbiology week 8: Acellular Microbes, Viruses and Prions

Learning Objectives

  • Describe basic structures: viruses, viroids, satellites, prions.

  • Understand virus classification.

  • Describe 5 steps of all viral life cycles related to infection and disease.

  • Differentiate between lysogenic and lytic cycles.

  • Explain virus diagnosis methods.

  • Describe prion infection mechanisms and related diseases.

Acellular Agents

  • Viruses: Nucleic acid (DNA/RNA) + protein coat (capsid). Some have lipid envelopes.

  • Viroids: Circular RNA molecules with no protein coats; infect plants.

  • Prions: Misfolded proteins without nucleic acids; propagate by inducing misfolding in normal proteins.

  • Satellites: Incomplete viral agents needing helper viruses for replication.

Virus Characteristics

  • Submicroscopic, obligate intracellular parasites.

  • Infect various hosts: animals, plants, bacteria (bacteriophages).

  • Exhibit some characteristics of life but lack independent metabolism and cellular structure.

Virus Significance

  • Major cause of infectious diseases in humans/animals.

  • Emerging viruses pose ongoing threats.

  • Serve therapeutic applications (gene therapy, cancer targeting).

  • Critical ecological roles in nutrient cycling and horizontal gene transfer.

Virus Taxonomy

  • Family: (-viridae) based on genome type, structure, replication strategy.

  • Genus: (-virus) more specific group within a family.

  • Species: common name for the individual virus.

  • Subtypes/strains: variants within a species.

Viral Structure

  • Nucleocapsid: Core structure with nucleic acid and capsid.

  • Capsid: Protects genome, formed from protein subunits called capsomeres.

  • Envelope: Lipid membrane from host cell, aids in attachment and entry.

Viral Life Cycle

  1. Attachment: Virus binds to specific host cell receptors.

  2. Entry: Membrane fusion/endocytosis; uncoating releases nucleic acid.

  3. Biosynthesis: Host machinery produces viral components.

  4. Assembly: New virions develop from genome and proteins.

  5. Release: Through cell lysis (non-enveloped) or budding (enveloped).

Infections by Bacteriophages

  • Virulent (Lytic) Phages: Rapidly lyse host cells.

  • Temperate Phages: Can replicate lytically or enter dormant lysogenic state.

Prions and Related Diseases

  • Arise from misfolded normal proteins, leading to neurodegenerative diseases.

  • Examples: BSE, Scrapie, Creutzfeldt–Jakob disease.

  • Propagation via misfolded proteins inducing normal proteins to misfold, leading to brain damage and dysfunction.