Viruses and Acellular Agents

1. Viruses and Acellular Agents MICROBIOLOGY LECTURES Sofronio Agustin Professor

2. Lesson Objectives: At the end of the lesson, the students should be able to: • Describe the structures and components of viruses, viroids and prions. • Describe the mechanism of infection and replication of bacteriophages and animal viruses. • Understand association between viruses and cancer and the phenomenon of viral latency and persistent infections.

3. Basic Terminology • Viruses are obligate intracellular parasites and require living host cell genetic and metabolic machinery for reproduction. • Viral genomes contain either DNA or RNA, not both. • Nucleic acid enclosed in a protein coat called Capsid. • Some are surrounded by membrane layer called Envelope, which may contain integral proteins such as Spikes. • Tropism – property of viruses to infect specific host cells; determined by specific host receptors and viral glycoproteins.

4. Viruses and Bacteria Compared

5. Virus Sizes and Shapes

6. Helical Viruses

7. Enveloped Helical Virus

8. Polyhedral Viruses An ICOSAHEDRON is a polygon with 12 triangular facets and 12 corners

9. Enveloped Icosahedral Virus

10. Complex Viruses • Bacteriophages have icosahedral heads, sheathed tail and tail fibers for host cell attachment. • Morphology akin to NASA’s Lunar Lander.

11. Viral Taxonomy • ICTV = International Committee on Taxonomy of Viruses • Family names end in –viridae e.g. Herpesviridae • Genus names end in -virus e.g. Papillomavirus • Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common English names are used for species e.g. Measles, Mumps, Rabies. • Subspecies are designated by a number e.g. HSV-1

12. Viral Taxonomy • Herpesviridae • Herpesvirus • Human herpes virus 1, HHV 2, HHV 3 • Retroviridae • Lentivirus • Human Immunodeficiency Virus 1, HIV 2

13. Electron Microscopy

14. Bacteriophage Culture • Viruses must be grown in living cells. • Bacteriophages form plaques on a lawn of bacteria.

15. Tissue Culture • Animal and plants viruses may be grown in cell culture. • Continuous cell lines may be maintained indefinitely.

16. Cytopathogenic Effects

17. Embryonated Egg Cultures • Animal viruses may be grown in living animals or in embryonated eggs. • Usually used in conjunction with vaccine production e.g. Flu vaccine • Some people who are allergic to eggs show sensitivity to these vaccines.

18. Virus Identification • Cytopathogenic effects (CPE) as seen in Tissue Culture. • Fluorescent Antibody Techniques (DFA or IFA) • Serological tests – to detect antibodies against viruses in a patient (4-fold rise in titers) • Antigen tests – rapid screening e.g. ELISA and Latex Agglutination tests. • Nucleic Acid Analysis – e.g. Western blot, Gene Probes, PCR – Polymerase Chain Reaction.

19. Stages of Replication in Lytic Phages • Attachment - phage attaches by tail fibers to host cell • Penetration - phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell • Biosynthesis - production of phage DNA and proteins • Maturation - assembly of phage particles • Release - phage lysozyme breaks cell wall

20. Replication Cycles in Phages • Lytic cycle - Phage causes lysis and death of host cell • Lysogenic cycle - Prophage DNA incorporated in host DNA; when activated becomes lytic.

21. Lytic Cycle of T-even Phages

22. Lytic Cycle of T-even Phages

23. One Step Growth Curve

24. Lysogenic Phages

25. Specialized Transduction

26. Replication of Animal Viruses • Attachment - Viruses attaches to cell membrane • Penetration - By endocytosis or fusion • Uncoating - By viral or host enzymes • Biosynthesis - Production of viral nucleic acid and proteins • Maturation - Nucleic acid and capsid proteins assemble • Release - By budding (enveloped viruses) or rupture (non-enveloped viruses)

27. Attachment , Penetration and Uncoating

28. Replication of DNA Viruses

29. Replication of RNA Viruses

30. Replication of Retroviruses

31. Release of Enveloped Viruses

33. Viruses and Cancer • Activated oncogenes transform normal cells into cancerous cells. • Transformed cells have increased growth, loss of contact inhibition, tumor specific transplant and T antigens. • The genetic material of oncogenic viruses becomes integrated into the host cell's DNA.

34. Oncogenic Viruses • Oncogenic DNA Viruses • Adenoviridae • Heresviridae • Poxviridae • Papovaviridae • Hepadnaviridae • Oncogenic RNA viruses • Retroviridae • Viral RNA is transcribed to DNA which can integrate into host DNA • HTLV 1 • HTLV 2

35. Latent and Persistent Viral Infections • Latent Viral Infections • Virus remains in asymptomatic host cell for long periods • Cold sores, shingles • Persistent Viral Infections • Disease processes occurs over a long period, generally fatal • Subacute sclerosing panencephalitis (measles virus)

36. Prions • Infectious proteins • Inherited and transmissible by ingestion, transplant, & surgical instruments • Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease • PrPC, normal cellular prion protein, on cell surface • PrPSc, scrapie protein, accumulate in brain cells forming plaques

37. Prion Infection Model

38. Plant Viruses and Viroids • Plant viruses enter through wounds or via insects • Viroids are infectious RNA; potato spindle tuber disease

43. Human Viruses Single-stranded DNA, nonenveloped viruses • Human parvovirus • Fifth disease • Anemia in immunocompromised patients

44. Human Viruses • Mastadenovirus • Respiratory infections in humans • Tumors in animals Double Stranded DNA Nonenveloped Viruses

45. Human Viruses • Papillomavirus (human wart virus) • Polyomavirus • Cause tumors, some cause cancer Double Stranded DNA Nonenveloped Viruses

46. Human Viruses • Orthopoxvirus (vaccinia and smallpox viruses) • Molluscipoxvirus • Smallpox, molluscum contagiosum, cowpox Double Stranded DNA Nonenveloped Viruses