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MICR 201 Microbiology for Health Related Sciences
Lecture 6: Viruses, viroids, and prionsEdith Porter, M.D.
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Lecture Outline
Viruses▪ General characteristics▪ Viral structure▪ Taxonomy▪ Isolation, cultivation and identification▪ Viruses and disease
Viroids Prions
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Viruses
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 chemicalsOR
Exceptionally simple living organism
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Size of viruses in comparion with bacteria and eukaryotic cells
Bacteria
Prions are not viruses
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
Bacteria and viruses compared
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
Main Appearance Forms of Viruses
Polyhedral (spherical, icosahedral)
Helical (filamentous)
Complex
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
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
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
Fits only to specific structures (receptors) on the
host cell
Mastadenovirus
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
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
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)
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
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
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 VVV V
VV
V
V
V
DEAD
V
Attachment Penetration Biosynthesis Maturation and
assembly Host lysis and virion
release
Lytic cycle of virus multiplication (1)
VV V
VV V
VV
VV
V
DEAD
V
Lytic cycle of virus multiplication (2)
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
VV
Lysogenic and lytic cycle of bacteriophage l in E. coli
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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
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Specialized transduction
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
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
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Entry and exit of animal viruses
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Bacteriophage and animal virus multiplication compared
Retroviruses
RNA viruses Include HIV Carry reverse transcriptase
RNA-dependent DNA polymerase Synthesize DNA from RNA Used in molecular biology (RT-PCR)
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Multiplication of a retrovirus
The course of viral diseases
http://pathcuric1.swmed.edu/PathDemo/gbp3/gbp340.jpg
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)
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
Viroids
RNA only Short piece of naked
RNA RNA does not code for
protein Similarities between
introns and viroids Often found in plant
diseases
Prions
Protein only 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
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Multiplication of prions
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