MICR 201 Microbiology for Health Related Sciences

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MICR 201 Microbiology for Health Related Sciences

• Lecture 6 Viruses, viroids, and prions
• Edith 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
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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

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

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Bacteria and viruses compared
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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

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Main Appearance Forms of Viruses

• Polyhedral (spherical, icosahedral)
• Helical (filamentous)

• Complex

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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

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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

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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
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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

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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
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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)

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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

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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

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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
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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
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Lytic cycle of virus multiplication (2)
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Lysogenic cycle 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
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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
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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

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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
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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
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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)

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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

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Viroids

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

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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
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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