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

2. Lecture Outline Viruses Viroids Prions 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
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 or RNA)
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. Main Appearance Forms of Viruses
Polyhedral (spherical, icosahedral)
Helical (filamentous)
Complex

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. Fits only to specific structures (receptors) on the host cell
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 Hemagglutination
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
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 V V
DEAD V V V V

19. Lytic cycle 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. Lytic cycle of virus multiplication (2)

21. Lysogenic cycle of virus multiplication
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. Lysogenic and lytic cycle of bacteriophage l 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)

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

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 Normal protein variant exists (PrPc)
Proteinaceous infectious 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 Viroids Prions
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