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NAJRAN UNIVERSITY College of Medicine. Microbiology &Immunology Course Lecture No. 20. By. Dr. Ahmed Morad Asaad Associate Professor of Microbiology. Virus-cell interactions Replication Viruses multiply only in living cells The replication cycle consists of the following steps:

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College of Medicine

Microbiology &Immunology Course

Lecture No. 20


Dr. Ahmed MoradAsaad

Associate Professor of Microbiology


Virus-cell interactions

  • Replication
  • Viruses multiply only in living cells
  • The replication cycle consists of the following steps:
  • 1- Attachment (adsorption)
  • Viral replication starts with interaction of the virus with specific receptors on the cell surface
  • Receptor molecules differ for different viruses (viral tropism)
  • Antiviral antibodies can inhibit this process

2- Penetration (Engulfment)

  • Non-enveloped viruses penetrate the host cell by a process of phagocytosis forming a cytoplasmic vacuole that will dissolve releasing the nucleocapsid into the cytoplasm
  • Enveloped viruses penetrate the cell by fusion of the viral envelope with the cell membrane releasing the nucleocapsid into the cytoplasm

3- Uncoating

  • It is the physical separation of viral nucleic acid from its capsid
  • 4- Eclipse phase
  • During this period (some hours), several enzymes are produced by the host cells under control of viral genome
  • The cell metabolism is now directed to the synthesis of new viral particles
  • This phase ends with the formation of the infectious virus

5- Synthesis of viral components

  • Transcription of mRNA from viral nucleic acid
  • The virus utilizes cell components to translate mRNA into viral components
  • In DNA viruses:
  • mRNA is transcribed by host cell DNA dependant RNA polymerase
  • In RNA viruses:
  • Viral RNA acts as a mRNA (+ve sense or +ve strand virus)
  • Viral RNA should be transcribed to mRNA (-ve sense or –ve strand virus)

6- Assembly

  • Viral nucleic acid becomes surrounded by its capsid to form the mature virion (nucleocapsid)
  • 7- Release
  • Release of viruses to extracellular environment either by cell lysis or by budding through cell membrane leaving the host cell intact
  • Enveloped viruses mature by a budding process

Inclusion bodies:

  • They are virus specific structures produced in the course of viral replication. (Areas of viral assembly)
  • Inclusion bodies may be:
  • Intranuclear: e.g. herpes viruses
  • Intracytoplasmic: e.g. pox viruses
  • In both: e.g. measles virus
  • Their presence in the cells is diagnostic of:
  • Negri bodies in Rabies
  • Guarnieri bodies in small pox
  • Torr’s bodies in yellow fever

Pathogenesis of viral infections:

  • The outcome of virus-cell interaction may be:
  • Cell death or lysis
  • Abortive infection: No synthesis of new viral particles. Viral replication is blocked
  • Latent infection: The viral genome persist inside the host cell for a period. There will be flare-up of clinical disease
  • Transformation: The host cell will acquire new characters leading to rapid growth and malignancy

Viral infections may be:

  • Superficial: short incubation period – No invasion. E.g. Influenza viruses, Rota virus
  • Systemic: After viral entry, it will spread to local lymphoid tissues followed by viraemia and spread to target organs.
  • Examples:
  • Skin & M.M. in measles and chicken pox
  • Liver in hepatitis viruses
  • Meninges in enterovirus meningitis

Persistent: The virus will persist for a long period. Three types of persistent viral infections:

  • 1- Chronic infection: leading to chronic diseases as in HBV and HCV
  • 2- Latent infection: The virus persists silent for a period of time with intermittent flare-up. E.g., herpes
  • 3- Slow virus infection: A very long incubation period, gradual onset and progressive course. E.g., AIDS