Viruses and other acellular pathogens

• Acellular Pathogens


  Viruses, prions, etc.
• Viruses

  • What are they (and what aren't they)?
  • Virus structure and classification
  • Viral infection
  • Viral life cycles
  • SARS-COV-2
  • Other acellular pathogens: prions, etc.
• Viruses

  Somewhere between life and non-life
• Mimivirus

  • Largest known virus, has DNA, RNA, and proteins, can repair its own DNA
• Mimivirus is 800 nanometers across, with a genome of 1.2 million bases
• General virus structure

  • All have a nucleocapsid composed of protein coat (capsid) and nucleic acid
  • Some have a phospholipid bilayer membrane (viral envelope)
• Viral envelope

  Common in animal and plant viruses, portions of the infected host cell membrane, often replace host proteins with viral glycoproteins
• Coronavirus


  An enveloped RNA virus that is the cause of COVID, Severe Acute Respiratory Syndrome (SARS), etc.
• Nucleocapsid architecture

  • Two most common shapes: helical & icosahedral, both are regular, geometric shapes determined by the structure and orientation of proteins
• Tobacco Mosaic Virus

  • A helical virus composed of only a single RNA molecule and one type of protein
• Adenoviruses

  • Non-enveloped icosahedral viruses which are generally mild pathogens in humans, birds, etc. causing conjunctivitis and respiratory illnesses
• In both helical and icosahedral capsid architectures, only one or two types of protein are used, meaning the virus needs only one or two protein-coding genes
• Complex viruses


  • Have somewhat more intricate structures, like bacteriophages with an icosahedral head, helical tail, and fibers for attachment
• Viral genomes

  Very small
• Viral genomes

  Can be either DNA or RNA (or some use both at different stages in their life cycle)
• How viruses are classified

  • Based primarily on: capsid structure, enveloped or naked, type and structure of nucleic acid, DNA vs RNA, single-stranded vs. double-stranded

  
• Examples of viral classification

  • E. coli bacteriophages
• Viral infection - overview

  1. Adsorption

  2. Entry (either of nucleocapsid or of nucleic acid only)

  3. Virion replication (immediate viral replication, integration into host genome, or latent infection)

  4. Exit from cell

  
• Adsorption


  Specific interaction of viral surface with cell surface, main reason viruses only infect certain cell types
• Entry

  1. Injection of nucleic acid (e.g. T4 bacteriophage)
  2. Entry of entire nucleocapsid (most common in animal and plant viruses)
  3. Membrane fusion (only in enveloped viruses)
• Viral replication

  Replication of the nucleic acid

  Synthesis of capsid proteins

  Assembly of nucleocapsids
• Exit


  Cell lysis or budding
• Viral life cycles

  • Lytic

  Lysogenic

  
• Bacteriophages

  Can contribute to the pathogenicity of bacteria
• A bacteriophage may be the trigger that makes the common and generally harmless Neisseria meningitidis invade the blood stream, cross the blood-brain barrier and lead to meningitis

  
• The carriage of virulence determinants by phages is not an uncommon situation in bacterial pathogens, as is the case in both cholera and diphtheria

  
• Life cycle of viruses with animal hosts (influenza)

  • Possible effects of animal virus infection of host cells

  
• Other acellular pathogens

  • Viroids - strands of self-replicating RNA that can infect plants

  Satellite RNAs - pathogenic RNAs that require a 'helper' virus to replicate

  Prions - infectious proteins discovered in 1982, cause Transmissible Spongiform Encephalopathy