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Microbiology

Microbiology. Introduction to Viruses Chapter 6. Part I . Viral Characteristics, Classification & Evolution . Viral Characteristics . Viruses – three basic forms Complex Poxviruses No capsid DNA surrounded by core membrane Bacteriophages Complex capsid head and tail structures

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Microbiology

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  1. Microbiology Introduction to Viruses Chapter 6

  2. Part I Viral Characteristics, Classification & Evolution

  3. Viral Characteristics • Viruses – three basic forms • Complex • Poxviruses • No capsid DNA surrounded by core membrane • Bacteriophages • Complex capsid head and tail structures • Naked or non-enveloped • Capsid which contains DNA or RNA • Enveloped • Outer membrane • Capsid which contains DNA or RNA

  4. Viral Structure Enveloped Naked Complex Poxvirus Envelope Outer Membrane Core Membrane Helical Capsid Plum poxvirus Helical Capsid Mumps DNA/RNA Capsid Icosahedral Capsid SS RNA Polio virus Sheath Tail Fibers Icosahedral Capsid Herpesvirus HIV Icosahedral Capsid DS DNA Papillomavirus Bacteriophage

  5. Viral Size

  6. Viral Genome Size • Viruses carry only genes necessary to instruct the host cell to make new viruses • For Comparison • Viral: 4 genes to hundreds of genes • E. Coli: 4,000 genes • Human Cell: 30,000 to 40,000 • BOTTOM LINE: viral genes invade host cells and redirect their activity

  7. Viral Characteristics • Viruses are known to infect every cell type • Obligate intracellular parasites • Namometers in size • Acellular • No metabolic enzymes • No machinery for protein production • Have instructions for their own reproduction, but not outside of host cell

  8. Viral Evolution • Two theories of viral evolution • Loose genetic material released by host cells that developed or obtained a protective coating and the ability to infect host cells • Once cells that kept only what was needed to infect host cells and redirect their own reproduction

  9. Viral Evolution • Viral Evolution • Some viruses interact with the genetic material of the host cell • Can carry genes or gene fragments from one host to another • They have had an integral role in the evolution of bacteria, archaea, & eukarya

  10. Viral Classification • Viruses are divided into families based on • Nature of nucleic acid • DNA or RNA • Type of capsid • Presence of absence of envelope

  11. DNA Viruses Smallpox Flu Chickenpox Shingles Herpes simplex Epstien Barr HPV HPV 16/18 Cancer Hepatitis B

  12. RNA Viruses

  13. Viral Host Range • Tropisms – viral tissue specificities • Restricted • Human liver cells – hepatitis B • Intermediate • Intestinal and nerve cells of primates - poliovirus • Broad • Various cells of all mammals - rabies

  14. Part II Viral Life Cycles

  15. Adsorption/Docking Specific interactions Penetration Receptor mediated endocytosis Vesicle fuses with lysosome Uncoating Enzymes in lysosome digest capsid, envelope (if present) and release viral genome Replication Copies of viral genome Copies of viral proteins Assembly Viral capsid reforms Packaging of genome Release Budding taking pieces of host cell membrane Lytic host cell burst open releasing many virus particles Viral Life Cycle in Brief

  16. Host Cell Receptors Step 1) Adsorption/docking Specific interaction between host cell receptors and viral proteins on envelope or naked capsid Step 2) Penetration – 2 mechanisms Viral particle is taken in by receptor mediated endocytosis or the host cell membrane fuses with the viral envelope releasing naked capsid to cytoplasm (see page 170 text) Step 3) Uncoating - 2 mechanism Internal vesicle fuses with lysosyme where enzymes digest envelope and capsid releasing viral genome. Or naked capsid is digested using proteases in the host cytoplasm. Either way the result is release of a free viral genome

  17. Step 4) Replication The mechanism of replication differs with genome type. (DNA vs. RNA genomes) However, this step requires the synthesis of the same basic components…copies of the viral genome and synthesis of many viral proteins Step 5 Step 6) Release Budding, exocytosis or host cell lysis (Lysis not shown) Step 5 Step 5) Assembly Viral proteins are inserted into host cell membrane and the nucleocapsid is formed from new genome copies and capsomers (proteins which make up the capsid)

  18. Virion Concentration • Number of virus released is variable • Contributing factors • Health of host • Size of virus • Examples • 3,000 to 4,000 poxvirus • 100,000 from poliovirus

  19. A successful viral life cycle included invading the host cell, making copies of many new viral particles, release of the viral particles from the cell and finally the transmission of the viral particles to a new host. Many virus particles are readily transmitted through respiratory droplets. Other means of transmission are ingestion of viral particles (enteric viruses) and blood to blood transmissions.

  20. DNAVirusLife Cycle Made by host RNA polymerase DNA viruses are targeted to the nucleus where they can potentially stably integrate into the host cell genome. The viral DNA may also become an extrachromosomal particle known as an episome. A prominent feature of this family is periods of latency followed by recurrent infections Chicken pox to shingles Reactivation of herpes New wart growth by HPV Replicated viral DNA made by host DNA polymerase

  21. DNA Viral Replication • DNA virus • Viral DNA targeted to host cell nucleus • Host DNA polymerase copies viral genome • Host cell RNA polymerase makes viral mRNA to make viral proteins • Host ribosomes synthesize viral proteins

  22. RNA Viral Life Cycle

  23. Negative Sense RNA Viral Replication No Integration into host genome. Virus uses viral enzymes and host machinery to replicate and complete its life cycle Host ribosomes used to make viral proteins RNA dependent viral RNA synthesis vRNA polymerase vRNA polymerase CCCGGAUAUCCGGAAAU GGGCCUAUAGGCCUUUA Note: this sequence is the same as the original negative sense genome strand Makes many (+) sense master templates GGGCCUAUAGGCCUUUA GGGCCUAUAGGCCUUUA GGGCCUAUAGGCCUUUA GGGCCUAUAGGCCUUUA GGGCCUAUAGGCCUUUA Makes many (-) sense genome copies (+) (-)

  24. Orthomyxoviruses • Characteristics of envelope proteins • Hemagglutinin (H) • Glycoprotein spikes • Used for attachment and entry into host • Targets ciliated cells of respiratory mucosa • Causes rapid destruction of these cells – inflammation • Genetic mutations (alterations) in these proteins is rare • Major component recognized by immune response • Genetic mutations in these proteins is common • Neuraminidase (N) • Enzyme that breaks down respiratory mucus • Assists in fusion to host cell

  25. Orthomyxoviruses – Antigenic Drift • Antigenic drift • Genetic change of glycoproteins through mutations • Decreased ability of host immune memory cells to bind to them • Drifting events can be common in many types of viruses

  26. Orthomyxoviruses – Antigenic Shift • Antigenic Shift • Viruses which infect two separate hosts infect a common host • Viral packaging can randomly produce hybrids • Hybirds produce glycoproteins which are not recognizable by your immune system • Source of many flu pandemics Note: antigenic drift and shift are not required to make Influenza deadly. Minor genetic alterations in other viral genes, which occur during the normal replication process can be the difference between a virulent stain and a lethal one.

  27. Retroviral Life cycle

  28. Retroviral RNA Virus Replication • Retrovirus • Reverses the central dogma of biology • Converts RNA into ds DNA • Stable integration of viral DNA into host genome possible • Uses viral reverse transcriptase • Viral enzyme Coded for on viral genome DNA Polymerase Reverse Transcriptase dsDNA ssDNA vRNA

  29. In chronic infections the virus continues to multiply and be released slowly. Latent infections require viral integration into host genomes or the formation of episomes. Oncogenic transformation occurs when viral genes usurp controls regulating host cell growth. These genes are known as viral oncogenes and they stimulate abnormal host cell growth and cancer formation

  30. Part III Cytopathic Effects

  31. Enveloped Virus & Cell Death? • Lytic cycles lead to cell death • What about budding cycles? • Most active viral infections are ultimately lethal to the cell due to accumulated damage • Permanent shutdown of host metabolism • Changes to host gene expression • Destruction of cell membranes • Toxicity of virus components • Release of lysosomes

  32. Cytopathic Effects CPE’s • Viral induced damage alters microscopic cell appearance • Gross changes in shape & size • Inclusion bodies • Compacted masses of viruses • Damaged organelles • Disrupt cohesive junction • Syncytia • Virus ability to fuse membranes • Causes large multinucleate cells

  33. CPE

  34. Viral Culture Techniques • Cell Culture • Primary vs. immortalized culture • Example cell culture systems • Embryonic kidney • Fibroblast • Bone marrow • heart • Embryonic Chick Cells • Live Animals

  35. Part IV Prions Transmissible Spongiform Encephalopathies

  36. Prions • Prions • Proteinaceous Infectious Particle • No DNA or RNA • Transmission • Genetically inherited • Acquired by ingesting contaminated tissues • Different names in different organism • Humans – Creutzfeld-jakob disease • Cows – mad cow disease • sheep and goats - scrapie

  37. Prions • Symptoms • Long latent periods before symptoms of brain degeneration appear • In all conditions the brain progressively deteriorates and the patient loses motor coordination along with sensory and cognitiveability • See pathology effects page 781 text

  38. Prions

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