Microbiology Lecture

• Host range


  Most viruses infect specific types of cells of only one host species, but rarely viruses cross the host-species barrier, thus expanding their host range
• Organisms susceptible to viral attack


  • Bacteria (bacteriophages)
  • Plants
  • Fungi

  
• Virus shapes and sizes

  • Helical
  • Polyhedral
  • Spherical
  • Complex

  
• Intracellular state (Viruses)


  Capsid removed, the virus exists as nucleic acid
• Extracellular state (Virions)


  Protein coat (capsid) surrounding nucleic acid, also called nucleocapsid, some have phospholipid envelope, outermost layer provides protection and recognition sites for host cells
• Capsid Morphology


  • Capsids provide protection for viral nucleic acid and means of attachment to host's cells, composed of proteinaceous subunits called capsomeres
• Viral Envelope


  Composed of phospholipid bilayer and proteins, some proteins are virally coded glycoproteins
• Spikes


  Carbohydrate-protein complexes that project from the surface of the envelope, used as a means of identification
• Genetic Material of Viruses

  • DNA viruses
  • RNA viruses

  
• Strands of Viral Genetic Material

  • Double-stranded DNA (dsDNA)
  • Single-stranded DNA (ssDNA)
  • Double-stranded RNA (dsRNA)
  • Single-stranded RNA (ssRNA)

  
• Types of Viral Hosts

  • Animal viruses
  • Plant viruses
  • Bacteriophages

  
• General Morphology of Viruses

  • Helical
  • Polyhedral
  • Spherical
  • Complex

  
• Morphology of a Helical Virus


  • Viral nucleic acid is found within a hollow, cylindrical capsid that has a helical structure

  
• Morphology of a Polyhedral Virus


  • The capsid is in the shape of an icosahedron, the capsomeres of each face form an equilateral triangle
• Morphology of a Spherical Virus


  • Helical viruses enclosed in an envelope, contain spikes made of glycoproteins used for adhering and entering host cells

  
• Morphology of a Complex Virus


  • Have capsids of many different shapes, several coats around the nucleic acid and no easily identifiable capsid, bacteriophages have polyhedral capsids (heads), attached to helical tails with tail fibers

  
• Families of Viruses that Affect Humans

  • Adenoviruses
  • Herpesviruses
  • Poxviruses
  • Papovaviruses
  • Hepadnaviruses
  • Retroviruses

  
• Isolation, Cultivation, and Identification of Viruses

  Viruses must be grown in living cells, bacteriophages form plaques on a lawn of bacteria, plaque method counts number of holes (plaque forming units), animal viruses may be grown in living animals or embryonated eggs, animal and plant viruses may be grown in cell culture

  
• Cytopathic effects


  Viruses infecting a monolayer sometimes cause the cells of the monolayer to deteriorate as they multiply
• Virus Identification Methods

  • Cytopathic effects
  • Serological tests (detect antibodies, use antibodies to identify viruses)
  • Nucleic acids (RFLPs, PCR)
  • Novel methods such as Biophotonics

  
• Viral Replication

  • Obligate intracellular parasites using host cell machinery, limited number of genes encode proteins for capsid formation, viral nucleic acid replication, and movement of virus into and out of cell, two mechanisms: lytic cycle and lysogenic cycle

  
• Lytic Cycle of Virulent Bacteriophage

  1. Attachment

  2. Penetration

  3. Biosynthesis

  4. Maturation

  5. Lysis

  
• The Lysogenic Cycle


  Phage infects bacterium and inserts its DNA into the bacterial chromosome, allowing the phage DNA (now called a prophage) to be copied and passed on along with the cell's own DNA

  
• Cancer


  Uncontrolled mitotic divisions, benign vs. malignant tumors, important characteristics: rapid cell division, loss of anchoring junctions and contact inhibition (metastasis), dedifferentiation of cells
• Viruses and Cancer


  The genetic material of oncogenic viruses becomes integrated into the host cell's DNA, conversion of proto-oncogenes to oncogenes transforms normal cells into cancerous cells, oncogenic viruses are responsible for ~10% of human cancers

  
• Oncogenic Viruses

  • Adenoviruses
  • Herpesviruses
  • Poxviruses
  • Papovaviruses
  • Human papilloma viruses (HPV)
  • Hepatitis B virus (HBV)
  • Human T cell leukemia viruses (HTLV-1, HTLV-2)

  
• Viroids


  • Extremely small, circular pieces of RNA that are infectious and pathogenic in plants, similar to RNA viruses but lack a capsid, may appear linear because of hydrogen bonding, viroidlike agents affect some fungi

  
• Prions


  • Proteinaceous infectious agents, cellular Prion Protein (PrPC) is normal and functional, prion PrP causes cellular PrP to refold into prion PrP, prion diseases are spongiform encephalopathies with large vacuoles forming in the brain, transmitted by ingestion, transplantation, or contact of mucous membranes with infected tissues, destroyed by incineration or autoclaving in concentrated sodium hydroxide