Chapter 13: Virus of Bacteria

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Chapter 13Virus of Bacteria
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Important Point
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What is a Phage?

• The agents were called filterable viruses Virus
  means poison, a term that once had been applied
  to all infectious agents. With time, the
  adjective filterable was dropped and only the
  word virus was retained.
• Bacteriophage were originally described as a
  macroscopic phenomenon that was slightly
  different from the poisoning of a plant or
  animal.
• Instead, what was observed was the destruction of
  a bacterial culture.
• People werent all that sure what a bacterium was
  so the destruction was seen more as an eating
  of the culture (by an otherwise unseen agent)
  rather than a poisoning.
• The filterable agent in this case was described
  as an eater of bacteria, or Phage from Greek
  Bacteriophage Bacteria Eater.

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What is a Phage?

• The agents were called filterable viruses Virus
  means poison, a term that once had been applied
  to all infectious agents. With time, the
  adjective filterable was dropped and only the
  word virus was retained.
• Bacteriophage were originally described as a
  macroscopic phenomenon that was slightly
  different from the poisoning of a plant or
  animal.
• Instead, what was observed was the destruction of
  a bacterial culture.
• People werent all that sure what a bacterium was
  so the destruction was seen more as an eating
  of the culture (by an otherwise unseen agent)
  rather than a poisoning.
• The filterable agent in this case was described
  as an eater of bacteria, or Phage from Greek
  Bacteriophage Bacteria Eater.

The word bacteriophage or phage that infect
bacteria is both singular and plural when
referring to one type of virus. The word phages
is used when different types of phages are being
referenced.
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Virus Architecture
Minimally, a virus is a proteinaceous carrier of
nucleic acid.
Many viruses are more complicated than that, such
as having a lipid envelope surrounding the
protein capsid.
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Terms Describing Virions

• Virion is another name for virus particle.
  Virions are infectious meaning that they can
  deliver their nucleic acid to the cytoplasm of a
  susceptible cell (which for phages would be a
  bacterium).
• Capsid is the the protein coat that surrounds the
  nucleic acid and defines a virus as a virus.
• Capsids are made up of individual proteins called
  capsomers.
• The virion particle consists, minimally, of
  protein and nucleic acid which together is called
  a Nucleocapsid.
• Many particularly animal viruses have lipid
  bilayers surrounding the nucleocapsid those
  viruses are described as Enveloped.
• Non-enveloped viruses are described as Naked.
• In enveloped viruses the envelope makes initial
  contact with cells and subsequent interaction
  with the cell surface is mediated by envelope
  proteins (proteins found in or on the envelope
  lipid bilayer).

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Virus Architecture
Note spikes projecting from protein capsid
surrounding nucleic acid.
Note spikes projecting from lipid
envelope-surrounding capsid (which in turn
surrounds the nucleic acid).
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Virion Attachment Organs

• Some virions are isometric they have a fully
  symmetrical capsids, almost spherical.
• Isometric virions attach to cells via Attachment
  Proteins, a.k.a., Spikes which are proteins that
  symmetrically project from their virions.
• Spikes project from capsids in Naked viruses.
• Spikes project from envelopes in Enveloped
  viruses.
• Some virions are helical with attachment proteins
  at end or ends.
• Most phages have tailed virions which sort of
  combine the morphology of isometric and helical
  virions, with the isometric part called a head
  and the helical part called a tail.
• Attachment is made at the end of the tail
  opposite the head, and often is mediated by thin
  feelers called tail fibers.

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Not-Complex Virions
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Complex (Tailed) Phage Virion
Note that this head actually is elongated top to
bottom rather than isometric.
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Virion Size
Big
Small
Medium
Even Smaller
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Viral Genomes
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
http//www.virology.net/Big_Virology/BVFamilyGenom
e.html
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Viruses are Not Cells
Smaller
Bigger
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Some Important Phages
Here complex means tailed
Note various genome architectures.
An example of Lysogenic Conversion.
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Lytic Infection Strategy
This is the productive-infection strategy
followed by most phages including all tailed
phages.
Phages that are obligately lytic are called
Virulent (vs. chronic or temperate).
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Chronic Infection Strategy
This is a process called Extrusion.
This is the strategy followed by filamentous
phages.
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Latent Infection Strategy
For phages this is called Lysogeny.
Only Temperate phages are able to display
lysogeny.
For phages this is called Lysogenic Conversion.
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Viral Infection Strategies
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Virulent Phage Life Cycle
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Chronic Phage Life Cycle
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Temperate Phage Life Cycle(s)
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Temperate Phage Life Cycle(s)
This is a process called Induction.
The prophage DNA can remain integrated
indefinitely.
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Adsorption Host Range

• Adsorption describes the virions attachment
  process.
• The virion contains proteins that recognize
  molecules found on the surface of cells (much as
  antibodies bind to antigens).
• Receptor Molecules (typically proteins) are
  molecules that cells make for various reasons
  (e.g., transport proteins) which viruses coopt
  for adsorption.
• Typically the Host Range of a virus is
  determined, at least in part, by the ability of
  the virus adsorption proteins to bind to host
  Receptor Molecules.
• Restriction-Modification systems can also limit
  phage host range (as well as biochemical
  incompatbilities).
• Through mutation (and other means) phages can
  overcome these host-range barriers, resulting,
  typically in a changed host range.
• Because of transduction, these changes in host
  range can allow gene exchange between even only
  distantly related bacteria.

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G. Eliave Institute, Tbilisi, Georgia
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G. Eliave Institute, Tbilisi, Georgia
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G. Eliave Institute, Tbilisi, Georgia
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G. Eliave Institute, Tbilisi, Georgia
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G. Eliave Institute, Tbilisi, Georgia
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G. Eliave Institute, Tbilisi, Georgia
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G. Eliave Institute, Tbilisi, Georgia
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