An Introduction to the Viruses


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An Introduction to the Viruses

  1. 1. Foundations in Microbiology Chapter 6 PowerPoint to accompany Fifth Edition Talaro Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  2. 2. An Introduction to the Viruses Chapter 6
  3. 4. Size of viruses
  4. 5. Naming viruses <ul><li>No taxa above Family (no kingdom, phylum, etc) </li></ul><ul><li>19 families of animal viruses </li></ul><ul><li>Family name ends in -viridae , Herpesviridae </li></ul><ul><li>Genus name ends in -virus, Simplexvirus </li></ul><ul><li>Herpes simplex virus I (HSV-I) </li></ul>
  5. 6. <ul><li>Family – Herpesviridae </li></ul><ul><li>Genus – Varicellovirus </li></ul><ul><li>Common name – chickenpox virus </li></ul><ul><li>Disease - chickenpox </li></ul>
  6. 7. capsids <ul><li>All viruses have capsids- protein coats that enclose & protect their nucleic acid </li></ul><ul><li>Each capsid is constructed from identical subunits called capsomers made of protein </li></ul><ul><li>2 types: </li></ul><ul><ul><li>helical </li></ul></ul><ul><ul><li>iscosahedral </li></ul></ul>
  7. 8. helical
  8. 9. icosahedral
  9. 10. icosahedral <ul><li>20-sided with 12 corners </li></ul><ul><li>Vary in the number of capsomers </li></ul><ul><li>Each capsomer may be made of 1 or several proteins </li></ul><ul><li>Some are enveloped </li></ul>
  10. 11. complex Fig 6.9a,c
  11. 12. 6 steps in phage replication <ul><li>adsorption – binding of virus to specific molecule on host cell </li></ul><ul><li>penetration –genome enters host cell </li></ul><ul><li>replication – viral components produced </li></ul><ul><li>assembly - viral components assembled </li></ul><ul><li>maturation – completion of viral formation </li></ul><ul><li>release – viruses leave cell to infect other cells </li></ul>
  12. 13. Fig 6.11
  13. 14. penetration
  14. 15. Bacteriophage assembly line
  15. 16. <ul><li>Not all bacteriophages lyse cells </li></ul><ul><li>Temperate phages insert their viral DNA into the host chromosome & viral replication stops at there until some later time. </li></ul><ul><li>Lysogeny - bacterial chromosome carries phage DNA </li></ul>
  16. 17. Host range <ul><li>Spectrum of cells a virus can infect </li></ul><ul><ul><li>cell has to have a specific structure (receptor) on its surface for viral attachment </li></ul></ul><ul><ul><li>cell has to contain all of the enzymes and materials needed to produce new virions </li></ul></ul><ul><li>May be one species or many </li></ul><ul><ul><li>HIV (only humans) vs rabies (many animals) </li></ul></ul><ul><li>May be one tissue or many within a host </li></ul><ul><ul><li>Hepatitis (liver) vs polio (intestinal & nerve cells) </li></ul></ul>
  17. 18. Differences between phage and animal virus replication <ul><li>Animal virus replication is more complex than phage replication because host cells are more complex. </li></ul><ul><li>Animal viruses cannot inject their DNA. </li></ul><ul><li>Lysogeny for phage, latency for animal viruses </li></ul>
  18. 19. Animal virus replication <ul><li>adsorption </li></ul><ul><li>penetration/uncoating of genome </li></ul><ul><li>duplication/synthesis </li></ul><ul><li>assembly </li></ul><ul><li>release </li></ul>
  19. 21. adsorption
  20. 22. penetration
  21. 23. Release by budding
  22. 24. Cytopathic effects- virus-induced damage to cells <ul><li>changes in size & shape </li></ul><ul><li>cytoplasmic inclusion bodies </li></ul><ul><li>nuclear inclusion bodies </li></ul><ul><li>cells fuse to form multinucleated cells </li></ul><ul><li>cell lysis </li></ul><ul><li>alter DNA </li></ul><ul><li>transform cells into cancerous cells </li></ul>
  23. 25. Cytopathic changes in cells
  24. 26. How do we grow viruses? Obligate intracellular parasites require appropriate cells to replicate.
  25. 27. Growing animal viruses <ul><li>live animals </li></ul><ul><li>bird embryos – chicken, duck; intact, self-supporting unit, sterile, self-nourished </li></ul><ul><li>cell culture </li></ul>
  26. 30. Other noncellular infectious agents <ul><li>prions - misfolded proteins, contain no nucleic acid </li></ul><ul><ul><li>cause spongiform encephalopathies – holes in the brain </li></ul></ul><ul><ul><li>common in animals </li></ul></ul><ul><ul><ul><li>scrapie in sheep & goats </li></ul></ul></ul><ul><ul><ul><li>bovine spongiform encephalopathies (BSE), aka mad cow disease </li></ul></ul></ul><ul><ul><ul><li>humans – Creutzfeldt-Jakob Disease </li></ul></ul></ul><ul><li>viroids - short pieces of RNA, no protein coat </li></ul><ul><ul><li>only been identified in plants, so far </li></ul></ul>
  27. 31. Diagnosis of viral diseases <ul><li>More difficult than other agents </li></ul><ul><li>Consider overall clinical picture </li></ul><ul><li>Take appropriate sample </li></ul><ul><ul><li>Infect cell culture- look for characteristic cytopathic effects </li></ul></ul><ul><ul><li>Screen for parts of the virus </li></ul></ul><ul><ul><li>Screen for immune response to virus (antibodies) </li></ul></ul>
  28. 32. diagnosis