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Foundations in Microbiology

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					An Introduction to the Viruses
            Chapter 6            1
                              Viruses
             “obligate intracellular parasites”

• Virus = (L. poison)                     • Infect all types of cells
      • Non-cellular                            •   Bacteria
           – infectious particles               •   Algae
      • Non-living                              •   Fungi
           – active or inactive                 •   Protozoa
      • Genetic parasites                       •   Plants
           – Use host cell machinery to         •   Animals
             replicate DNA or RNA




                                                                        2
Poxvirus                      Ebola Virus                      Vaccinia Virus
         Viruses = Ultramicroscopic
• Smallest infectious agents
  (< 0.2 m)
   – Parvoviruses (respiratory
     infections) = 20 nm

   – Poxviruses (smallpox) =
     250 nm

   – 2000 viruses/bacterial cell

   – 50 million/human cell



                                      3
            Viral Terminology
• Virion = infectious extracellular particle

• Capsid = outer coating

• Nucleocapsid = nucleic acid + capsid




                                               4
    Naked vs Enveloped Viruses




• Envelope = modified host cell membrane
• Spikes or peplomers = attachment of virus
• Pleiomorphic due to flexible envelope
                                              5
              Capsid = Protein Coat
• Encloses & protects nucleic
  acid (DNA & RNA)

• Composed of identical
  protein subunits = capsomers

• 2 types:
   – helical
   – iscosahedral




                                      6
                              Helical Capsids
•   Rod-shaped capsomers
    form helix

•   DNA or RNA = coiled
    inside

•   Naked helical (rigid)
     – Tobacco mosaic virus


•   Enveloped helical
    (flexible)
     – Influenza
     – Measles
     – Rabies



                                                7
Icosahedral Capsids




• 20-sided with 12 corners

• Vary in # of capsomers
                             • Naked
       • Poliovirus (32)
                                – Papillomavirus (warts)
       • Adenovirus (240)
                             • Enveloped
                                – Herpes simplex (cold sores)
• Capsid = sphere or cube                                8
               Fig. 6.10
Poxvirus   Mumps Virus   Herpesvirus     Adenovirus




 Phage     Rhabdovirus     HIV         Papillomavirus
                     Viral Core
•   DNA or RNA                • Carry genes for:
•   Single or double-stranded   – Viral capsid
•   Virus = genetic parasite    – Genetic material
                                – Host regulation
•   Hepatitis B: 4 genes
                                – Viral packaging
•   E. coli: 4000 genes
•   Human: 40,000 genes



                                                     10
              Classifying & Naming Viruses

• Latin root (descriptive) plus -viridae   • Rhabdovirus = bullet-shaped
   – Family name ends in -viridae          • Togavirus = cloak-like
      (Herpesviridae)
                                           • Adenovirus = infect adenoids
    – Genus name ends in -virus            • Lentivirus = slow, chronic
      (Varicellovirus)                                      infection
    – Common name (Chickenpox)             • Picornavirus = small RNA
                                           • Reoviruses = respiratory,
• Capsid type (helical or icosahedral)                      enteric viruses

• Enveloped or naked

• DNA (6 families) or RNA (13 families)

• Host cell (bacterial, plant or animal)

• Size & shape
              DNA vs RNA Viruses
DNA Viruses                                   RNA Viruses
• Poxviruses (smallpox)                       • Poliovirus
                                              • Rhinovirus (colds)
• Herpesviruses
                                              • West Nile virus
• Hepatitis B virus                           • Influenza virus
• Papillomavirus (warts)                      • Mumps
• Adenoviruses                                • Measles
                                              • Rabies
                                              • HIV (retrovirus)

 Important thing here is that both DNA and RNA viruses exist         13
                    Animal Virus Replication
1. Adsorption (specific,
     mediated by receptors, much of the
     time by receptor-mediated
     endocytosis)


2. Penetration &
   uncoating of genome

3. Duplication/synthesis

4. Assembly (also called
     maturation)


5. Release (budding, lysis)
                                               14
                              Host Range
• Tropism = Spectrum of cells a virus can infect
   – cell has specific structure (receptor) on its surface for viral attachment
             – HIV-1 (CD4 and chemokine receptor)
             – Epstein-Barr Virus (complement protein, CR2)
             – SARS (ACE-2 = angiotensin converting enzyme)

    – cell has to contain enzymes & materials needed to produce new virions
• May be one species or many
   – HIV (only humans) vs rabies (many animals)

• May be one tissue or many within a host
   – Hepatitis (liver) vs polio (intestinal & nerve cells)



                                                                              15
Phage Movie




              16
Retrovirus Movie




                   17
Adsorption




       Coronavirus (SARS)




             Adenovirus     18
Viral Penetration




 Herpesvirus:Endocytosis




    Mumps Virus: Fusion


                           19
2 Mechanisms of Entry
       Movie




                        20
                 Release by Budding




•Budding occurs by exocytosis
•Virions are slowly shed from cell
•Pox-infected cell releases 3000-4000 virions
•Poliovirus-infected cell releases 100,000
virions
                                                21
                    Cytopathic Effects:
              Virus-Induced Damage to Cells
1.   Changes in size & shape
2.   Nuclear & cytoplasmic inclusion bodies
3.   Cells fuse to form multinucleated cells
4.   Cell death & lysis
5.   Alter DNA
6.   Oncoviruses: transform cells into
     cancerous cells
        •    Epstein-Barr virus (lymphoma)
        •    Papillomavirus (cervival cancer)
        •    HTLV (leukemia)

Persistent infections: last weeks to years
Chronic latent state:
    Herpes (cold sores & genital herpes)
    Varicella-Zoster (chickenpox & shingles)
                                                22
 Growing Animal Viruses


1.   Live animals (In vivo)

2.   Bird embryos (chicken & duck)
       intact
       self-supporting unit
        sterile
       self-nourished

3. Cell culture (In vitro)


                                     24
  Infection & Plaque Formation
• Plaques = areas where virus-
  infected cells undergo lysis




                                 25
     Other Noncellular Infectious Agents
1.    Prions: misfolded proteins
     –    contain no nucleic acid
     –    cause spongiform encephalopathies
          = holes in the brain
     –    common in animals
          • Sheep & goats (Scrapie)
          • Cattle (Mad cow disease)
          • Humans – Creutzfeldt-Jakob
              Disease (beef) & Kuru
              (cannibalism)

2.   Satellite Viruses: Require helper virus
     to replicate
          • Adeno-associated viruses
          • Delta agent (RNA) needs HepB

3. Viroids: short pieces of RNA, no protein
     coat
    –    only been identified in plants, so far   26
            Most of the time based on
            symptomology


diagnosis




                                        27
                Importance of Viruses
• Medical: infectious disease    • Retroviruses (HIV-1)
                                         • Treatment of rheumatoid arthritis
   –   Colds
   –   Rabies                    •   Adenoviruses (common cold)
                                         • Cystic fibrosis
   –   AIDS
                                 • Poxviruses
   –   Ebola                             • Canarypox (cancer & rabies
                                           treatment)

• Agricultural: plant viruses    • Herpes viruses (cold sores &
                                   encephalitis)
                                         • Infect neurons & CNS
• Ecological: 10 million/ml or           • Treat tumors in brain
                                         • Parkinson’s disease
  270 million metric tons of
                                 • Vaccinia virus
  organic matter in seawater             • Vaccines for smallpox & rabies


• Research: genetic tools                                               28

				
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