Acellular and Prokaryotic Microbes

• the smallest infectious agent (about 20nm - 300nm in diameter) and contain only one type of nucleic acid.


  Virus
• infectious virus particle


  virion
• Protein shell which surrounds and protects the genome. It is built up of multiple

  (identical) protein sub-units called capsomers.


  capsid
• genome + capsid


  nucleocapsid
• Lipoprotein membrane which surrounds some viruses, derived from the plasma membrane of the host cell


  Envelope
• Proteins found in the envelope of the virion; usually glycosylated

  Glycoproteins
• MORPHOLOGY

  Seen in many filamentous and pleomorphic viruses

  helical morphology
• MORPHOLOGY

  Is characterized of many "spherical" viruses


  icosahedral morphology
• polyhedron having 20 faces

  isocahedral
• single molecule of nucleic acid

  monopartite genomes
• places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), Sense, and method of replication.


  classification
• adenoviruses, herpesviruses and poxviruses

  dsDNA viruses
• parvoviruses


  ssDNA viruses
• reoviruses


  dsRNA viruses
• picornaviruses, togaviruses


  (+)ssRNA viruses
• orthomyxoviruses, rhabdoviruses


  (-) ssRNA viruses
• retroviruses


  ssRNA-RT viruses
• hepadnaviruses


  dsDNA-RT viruses
• Viruses originated in the primordial soup and coevolved with bacteria and archaea


  Coevolution theory
• Viruses evolved from free-living prokaryotes that invaded other living organisms

  Retrograde Evolution Theory
• Viruses are pieces of host cell RNA or DNA that have escaped from living cells and are no longer under cellular control

  Escaped Gene Theory
• 3 types of origin of viruses theories

  coevolution theory, retrograde evolution theory, escaped gene theory
• • also known as adsorption
  • highly specific
  • The surface of the virion contains structures that interact receptors on the surface of the host cell.
  • It defines and limits the host species and type of cell that can be infected by a particular virus


  attachment
• the process whereby the virion enters the cell

  penetration
• • the protein coat of the virion dissociates
  • the viral genome is released into the cytoplasm

  uncoating
• 3 phases of biosynthesis

  early phase, genome replication, and late phase
• Transcription of viral mRNA and translation of a number of non-structural proteins takes place

  early phase
• Multiple copies of the viral genome are synthesized by a viral polymerase

  genome replication
• Transcription and translation of viral mRNA and synthesis of the structural “late” proteins which are needed to make new virions

  late phase
• • the protein self-assemble and a genome enters each new capsid
  • this takes place either in the nucleus or in the cytoplasm of the cell, or sometimes, just beneath the cell surface

  assembly
• the final stage of replication

  release
• 1. implantation of virus at the portal of entry
  2. local replication
  3. spread to target organs (disease sites)
  4. spread to sites of shedding of virus into the environment

  pathogenic mechanisms of viral disease
• A number of “entry-inhibiting” or “entry-blocking”

  drugs are being developed to fight HIV

  entry inhibitor
• Amantadine and rimantadine, have been

  introduced to combat influenza

  • These agents act on penetration/uncoating

  uncoating inhibitor
• By developing nucleotide or nucleoside analogues and deactivate the enzymes that synthesize the RNA or DNA once the analogue is incorporated

  reverse transcription
• Splices the synthesized DNA into the host cell genome

  integrase
• block attachment of transcription factors to viral DNA

  transcription
• Based on “antisense” molecules or ribozyme antivirals that have been developed to treat HIV infections

  translation
• it is used to treat selected patients with HIV infection

  protease inhibitors
• Two drugs zanamivir (Relenza) and oseltamivir (Tamiflu) that have been introduced to treat influenza

  release phase