MICR 201 Microbiology for Health Related Sciences

1. Lecture 6: Viruses, viroids, and prions Edith Porter, M.D. MICR 201 Microbiology for Health Related Sciences

2. Lecture Outline • Viruses • General characteristics • Viral structure • Taxonomy • Isolation, cultivation and identification • Viruses and disease • Viroids • Prions

3. Viruses

4. What is a Virus? • Virus is Latin for poison • Initially, viral disease described as disease that could be transmitted with poisonous fluid that had been passed through a “sterile” filter • Acellular infectious agent • Exceptionally complex aggregation of non-living chemicals OR • Exceptionally simple living organism

5. Size of viruses in comparion with bacteria and eukaryotic cells Bacteria Prions are not viruses

6. General characteristics of viruses • Obligatory intracellular agents • Require a cellular host for proliferation • Multiply inside living cells by using entirely the synthesizing machinery of the cell • Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells • Most viruses infect only specific types of cells in one host • Host range is determined by specific host attachment sites and cellular factors • Contain single type of nucleic acid (DNA orRNA) • Contain a protein coat • Some are enclosed by an envelope • Some viruses have spikes

7. Bacteria and viruses compared

8. Basic Viral Structure • Contain a single type of nucleic acid • Either DNA or RNA • Single or double stranded • Protein coat surrounding nucleic acid • Capsid • Composed of capsomer subunits • Protects • Vehicle for transmission • May contain a very limited number of enzymes • Virion: complete virus with nucleic acid and protein coat

9. Polyhedral (spherical, icosahedral) Helical (filamentous) Complex Main Appearance Forms of Viruses

10. Additional virus structures • Spikes • Glycoproteins projecting from surface • Can clump cells • Hemagglutination • Envelopes • Originate mainly from host membranes • Lipid, protein, carbohydrate • Some virus encoded proteins

11. Viral taxonomy • Based on nucleic acid composition, replication mode, morphology • Family names end in –viridae (Herpesviridae) • Genus names end in –virus (Herpesvirus) • Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species (Herpes simplexvirus) • Subspecies are designated by a number (Herpes simplexvirus 2) • Often abbreviated names • HSV • HIV • CMV

12. Viral hosts • In principle, any cell can be infected by a virus • A particular virus can infect only a specific host cell type (receptor mediated entry) • Prokaryotic cells • Bacterial viruses (phages) • Archaeal viruses • Eukaryotic cells • Fungal cells • Algae • Protozoa • Plant cell viruses • Insect cell viruses • Animal viruses Mastadenovirus Fits only to specific structures (receptors) on the host cell

13. How to Culture Viruses? • Need a living host cell • Bacteriophages • Bacteria • Plant virus • Plant cells • Animal viruses • Living animal • Embryonated eggs • Large scale production • Cell culture • diagnostics

14. Viral cytopathic effects in cell culture • Virus induced changes of eukaryotic cell morphology • Cell rounding • Cell aggregation • Inclusion bodies • Cell fusion • Transformation • Continous growth of eukaryotic cell • Can be used for diagnostic Normal Cell rounding

15. Virus identification • Cytopathic effects • Monkey kidney cells • Foreskin cells • Fibroblasts • Hemagglutination • Serology (look for patient antibodies) • PCR (polymerase chain reaction, detects virus specific nucleic acid sequences) • RFLP (restriction fragment polymorphism)

16. How to Quantify Viruses • Electron microscope • Count • Plaque Assay • Bacterial lawn • Add virus • Add agar on top to immobilize virus • Incubate • Count plaques • lack of bacterial growth where 1 virus had been

17. Viral multiplication : one-step growth curve • Eclipse is the period immediately after penetration during which not a single intact virus is present • In vitro, no new host cells are provided and number of virions decrease over time

18. Viral multiplication in bacteria V V • Typically double stranded DNA viruses • Lytic cycle • Rapid large scale production of viruses • Host cell lysis and death • Lysogenic cycle • Host cell survives • Viral genome incorporated into host cell genome • Replication with host cell • No active virion production V V V V V V V V V DEAD V V V V

19. Lyticcycle of virus multiplication (1) • Attachment • Penetration • Biosynthesis • Maturation and assembly • Host lysis and virion release V V V V V V V V V V V V DEAD

20. Lyticcycle of virus multiplication (2)

21. Lysogeniccycle of virus multiplication V V • Attachment • Penetration • Phage DNA integrates into bacterial host genome by recombination • Virus now: prophage • Host cell: lysogenic bacterium • Lysogenic conversion: bacterium produces virus encoded proteins • Prevent superinfection with similar phage • Some are toxins (e.g. diphteria toxin by C. diphteriae) • Switch to lytic cycle • can be induced by UV light • Specialized transduction: accidentally, bacterial host DNA is cut out too V V V

22. Lysogenicand lytic cycle of bacteriophagel in E. coli

23. Transduction • Virus serves as vector for bacterial DNA • During virus assembly a segment of bacterial DNA is accidentally packed into virus capsids • Specialized transduction: a segment of bacterial DNA along with the proper viral DNA • Generalized transduction: only bacterial DNA is packaged into the capsid

24. Specialized transduction

25. Animal viruses • DNA or RNA viruses • Single or double stranded • Negative or positive sense • Unique viral biosynthesis pathways • RNA viruses require enzymes not present in eukaryotic cells

26. Key steps in the multiplication of animal viruses • Attachment • Entry • Uncoating • Biosynthesis • Early genes for replication • Late genes for structural elements • Assembly (maturation) • Release • Host rupture: non-enveloped viruses • Budding: enveloped viruses

27. Entry and exit of animal viruses

28. Bacteriophage and animal virus multiplication compared

29. Retroviruses • RNA viruses • Include HIV • Carry reverse transcriptase • RNA-dependent DNA polymerase • Synthesize DNA from RNA • Used in molecular biology (RT-PCR)

30. Multiplication of a retrovirus

31. The course of viral diseases • Acute Infection • Unspecific: fever, muscle and joint aches • Specific: depend on target host cell • Latent Infections • Virus retreats in host cells • Herpesviridae in neurons (fever blisters) • Persistent Infections • Slow virus disease • Gradual increase of symptoms • Subacute sclerosing panenecephalitis after measles infection • Cancer • Chicken leukemia virus, Epstein Barr virus (lymphoma), HPV (cervix carcinoma) HBV (liver cancer) http://pathcuric1.swmed.edu/PathDemo/gbp3/gbp340.jpg

32. Caused by various virus genera Conjunctivitis Diarrhea Encephalitis “Flu” Influenza SARS Avian flu Hepatitis Diseases with virus specific symptoms Measles Rubella Herpes AIDS Examples for viral diseases

33. Viroids • RNA only • Short piece of naked RNA • RNA does not code for protein • Similarities between introns and viroids • Often found in plant diseases

34. Prions • Protein only • Proteinaceousinfectious particle • Infectivity can be reduced with protease treatment • Infects central nervous system • Normal protein variant exists (PrPc) • Prion protein (PrPsc) induces conformation change of normal variant and aggregation • Snow ball effect • Damage in central nervous system due to loss of cell function and inflammatory host response • Neurological disease • Mad cow disease (with limited human transmission) • Scrapie • Creutzfeldt-Jakob disease

35. Multiplication of prions

36. Important to remember • Viruses • Acellular, requires a host cell to mulitply • Protein capsid and 1 type of nucleic acid (RNA or DNA) • Spherical, helical and complex structure • Can have envelopes and carry a few enzymes • Lytic and lysogenic multiplication in bacteria • Uncoating and budding is part of animal virus multiplication • Viroids • RNA only • Plant diseases • Prions • Protein only • Neurological diseases