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Obligate Intracellular Parasite

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Title: Obligate Intracellular Parasite


1
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.)

2
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
3
Virus Relative Size (Small)
Note that Viruses are not cells Repeat after me,
Viruses are not cells And Antibodies are not
Antibiotics, either!
4
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

5
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

6
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

7
Virus Envelopes (Spikes)
8
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)

9
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

10
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)

11
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
12
Reovirus Spider
Note This is a disrupted reovirus showing
segmented dsRNA, not an intact virus that looks
like a spider!
13
Virion Morphologies
14
Naked Helical
15
Enveloped Filamentous
16
Naked Polyhedral
17
Enveloped Polyhedral
18
Enveloped Spherical (HIV)
19
Tailed Phages (complex)
20
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

21
Animal Virus Classification
22
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)

23
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.
24
Bacteriophage Lytic Cycle
25
Latent Period Burst Size
26
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

27
Phage Virion Maturation
28
Enveloped Virus Budding
29
DNA Enveloped Virus Replication
30
Minus-Strand RNA Enveloped
31
HIV Retrovirus
32
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

33
Plaquing Viruses
34
Cytopathic Effects
Cytopathic effects are harm viruses can do to
cells, short of totally destroying cells
35
Syncitium Formation
36
Syncitia and Inclusion Bodies
37
Embryonated Egg
38
Consequence of Virus Infection
39
Link to Next Presentation
40
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
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