Obligate Intracellular Parasite

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Obligate Intracellular Parasite

• Some bacteria are obligate intracellular
  parasites
• For example, Chlamydia spp.
• All viruses are obligate intracelluar parasites
• Obligate Intracellular Parasite means that
  these would-be replicators must locate and invade
  a cell in order to replicate
• Outside of cells viruses are nearly inert they
  dont even have cells (e.g., plasma membrane,
  ribosomes, cytoplasm, etc.)

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Virus (Definition)
Viruses are ...infectious agents of small size
and simple composition that can multiply only in
living cells of animals, plants and bacteria
plus fungi protozoa. Viruses are obligate
parasites that are metabolically inert when they
are outside their hosts. They all rely, to
varying extents, on the metabolic processes of
their hosts to reproduce themselves. The viral
diseases we see are due to the effects of this
interaction between the virus and its host cell
(and/or the hosts response to this
interaction). Encyclopedia Britannica
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Virus Relative Size (Small)
Note that Viruses are not cells Repeat after me,
Viruses are not cells And Antibodies are not
Antibiotics, either!
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Virions (Virion Particle)

• The Virion is what defines a virus as a virus
• A Virion is the extracellular state of a virus
• The job of Virions is to find new cells to infect
• As such, Virions are a durable state that is
  designed to attach to susceptible cells
• The Virion is then responsible for translocation
  of the virus genome into the cell
• The Virion consists of a DNA (or RNA) genome
  surrounded by Protein that, in turn, may be
  surrounded by a Lipid Bilayer(s)
• The Protein layer is called a Capsid
• The Lipid Bilayer is called an Envelope

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Capsid (Nucleocapsid)

• The Capsid is the protein shell surrounding the
  virus nucleic-acid genome
• A Nucleocapsid is that combination of protein
  capsid and nucleic-acid genome
• Capsids consist of multiple subunits of one or
  more types of protein
• Capsomere(s) capsid protein subunits
• For viruses with complex morphologies the capsid
  may consist of many (10s) different types of
  capsomer proteins, with additional proteins
  involved simply in capsid morphogenesis
• Capsids are responsible for nucleic-acid
  protection and penetration into cells

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Virus Envelopes (Spikes)

• In addition to the capsid, many animal viruses
  additionally posses Envelopes
• An envelope is a lipid bilayer that surrounds the
  nucleocapsid
• For enveloped viruses the envelope is also (in
  addition to the capsid) involved in nucleic-acid
  protection and penetration
• Without an envelope an otherwise enveloped virus
  is not mature nor infectious
• Enveloped proteins often have Glycoproteins
  (Spikes) projecting from their envelopes that are
  involved in virus infection
• For Non-Enveloped viruses the capsid is solely
  responsible for nucleic-acid protection and
  penetration

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Virus Envelopes (Spikes)
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Virus Proteins

• Are coded by Viral Genes (and made only while a
  virus is infecting a cell) They
• Protect the viral nucleic acid
• Attach to receptors on the outside of cells
• Penetrate cell membranes
• Replicate viral nucleic acid (proteins with this
  function are not associated with all virus types)
• Begin the program for virus replication (ditto)
• Modify the infected cell (beyond towards making
  new virus particles ditto)

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Viral Receptors

• Are coded by Host Genes (and are made regardless
  of whether a virus is present) They
• Often consist of membrane proteins
• But may also consist of carbohydrates attached to
  the outside of plasma membranes
• Do not exist for the sake of virus attachment but
  instead serve some function in host metabolism
• The Host Range of a virus often is determined by
  these molecules found on the outside of cells
• Host mutation to viral resistance can occur as a
  consequence of loss of specific viral receptors

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Virus Classification

• Viruses are classified
• According to their Genome Type
• Their Virion Morphology (as determined by
  electron microscopy)
• Their strategies of replication
• Their Serology (serotype), which is their
  reaction with specific antiserum
• (Increasingly) in terms of the sequence of their
  genomes
• International Committee on Taxonomy of Viruses
  (ICTV)

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Viral Genomes
http//www.virology.net/Big_Virology/BVFamilyGenom
e.html
One way to distinguish different types of viruses
(e.g., influenza virus from HIV) is in terms of
the characteristics of their nucleic-acid genomes
Nucleic-Acid Virus Genome
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Reovirus Spider
Note This is a disrupted reovirus showing
segmented dsRNA, not an intact virus that looks
like a spider!
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Virion Morphologies
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Naked Helical
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Enveloped Filamentous
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Naked Polyhedral
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Enveloped Polyhedral
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Enveloped Spherical (HIV)
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Tailed Phages (complex)
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Host Range vs. Tropism

• Another way that viruses can be classified is in
  terms of their host range
• Host Range is the range of species (strains, or
  varieties) of organisms that can serve as hosts
  for a given virus (also, narrow vs. wide)
• Similar to host range is a virus Tropism which
  is the range of tissue or cell types within a
  given host that a virus can infect
• Tropism is similar to Host Range because of the
  mechanisms controlling both Typically presence
  or absence of specific Viral Receptors or
  Peculiarities in a Cells Biochemistry

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Animal Virus Classification
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Animal Virus Classification
Fifth Disease virus destroys cells in bone
marrow that give rise to blood cells infected
children have a bright red rash on the cheeks
(slapped cheek rash) that may spread to the
trunk and extremities, low grade fever serious
danger to those having chronic hemolytic anemia
(ex. sickle cell anemia) virus can travel across
the placenta probably spread via respiratory
route. (http//www.austincc.edu/rohde/CHP1920.h
tm)

• 19th Century terminology for Childhood Rash
  Diseases
• Scarlet Fever (group A streptococci S. pyogenes)
• Rubeola (measles)
• Rubella (German measles)
• Epidemic pseudoscarlatina (type of sepsis???)
• Fifth disease (erythema infectiosum)

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Steps of Virus Replication

• Adsorption (attachment)
• Penetration (nucleic-acid release)
• Synthesis (of RNA and proteins, as well as DNA if
  employed)
• Maturation (assembly of virion)
• Release (lysis or chronic release, e.g., budding,
  with the latter coinciding with release for
  various enveloped viruses)

Caveat It is important to realize that variation
among viruses is between virus strains/species
any one kind of virus cannot replicate in
multiple ways, have more than one virion
morphology, or vary in genome type, etc.
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Bacteriophage Lytic Cycle
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Latent Period Burst Size
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Lambda Lytic vs.Lysogenic Cycles

• Phage ? is a Temperate phage
• Which means that it capable of displaying
  Lysogeny
• One consequence of lysogeny is lysogenic
  conversion (a.k.a., phage conversion) which is a
  change in bacterial phenotype upon lysogenization
• A number of bacterial virulence factors,
  including toxins, are coded by temperate
  Prophages including the Shigatoxin produced by
  lambdoid phage of E. coli O157H7

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Phage Virion Maturation
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Enveloped Virus Budding
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DNA Enveloped Virus Replication
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Minus-Strand RNA Enveloped
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HIV Retrovirus
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Tissue (Cell) Culture

• Culturing viruses within animals can be
  expensive, time consuming, difficult to observe,
  and difficult to do reproducibly
• Consequently, virus propagationtoday typically
  proceeds either within tissue culture or using
  animal approximations such as embryonated eggs
• Much modern experimental animal virology is done
  using tissue culture

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Plaquing Viruses
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Cytopathic Effects
Cytopathic effects are harm viruses can do to
cells, short of totally destroying cells
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Syncitium Formation
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Syncitia and Inclusion Bodies
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Embryonated Egg
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Consequence of Virus Infection
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Link to Next Presentation
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Acknowledgements
http//duke.usask.ca/misra/virology/slides/intro.
ppt http//pls.atu.edu/biology/biology/people/bisk
/genbio/ch20_lecture.ppt http//web.isoi.edu.pk/IS
OIWebPages/Faculty-n-TAs/awesterman/Powerpoint/Cam
pbell20-20Chapter2018.ppt http//www.gpc.peachn
et.edu/ccarter/micropresent/microlecschap13/micro
lecschap13.PPT http//duke.usask.ca/misra/virolog
y/slides/morb.ppt http//ww2.madonna.edu/smithe/Sm
ith20Web/Micro/Pages/Exam20II/virus2.ppt http//
www38.homepage.villanova.edu/john.friede/BIO20359
5/Viruses,20intro.ppt