Chapter 13 Virus of Bacteria

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Chapter 13 Virus of Bacteria Powered By Docstoc
					   Chapter 13:
Virus of Bacteria
Important Point:
                    “The agents were called filterable viruses… Virus
                     means „poison,‟ a term that once had been
                     applied to all infectious agents. With time, the
What is a Phage?
                     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 weren‟t 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 agents were called filterable viruses… Virus
                     means „poison,‟ a term that once had been
                     applied to all infectious agents. With time, the
                               “The word was dropped or
What is a Phage?
                     adjective filterable bacteriophage and only the
                             phage that infect bacteria is both
                     word virus was retained.”
                            singular and plural when referring
                    Bacteriophage were originally described as a
                              to one type of virus. The word
                     macroscopic phenomenon that was slightly
                              phages is used when different
                     different from the “poisoning” of a plant or animal.
                                 types of phages are being
                    Instead, what was observed was the destruction
                                        referenced.”
                     of a bacterial culture.
                    People weren‟t 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.”
                       Minimally, a
                         virus is a
Virus Architecture
                      proteinaceous
                     carrier of nucleic
                            acid.

                        Many viruses
                          are more
                        complicated
                          than that,
                           such as
                        having a lipid
                          envelope
                        surrounding
                         the protein
                           capsid.
                            Virion is another name for virus particle. Virions are
Terms Describing Virions     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).
                      Note spikes
                     projecting from
Virus Architecture
                     protein capsid
                      surrounding
                      nucleic acid.


                        Note spikes
                         projecting
                         from lipid
                         envelope-
                        surrounding
                       capsid (which
                           in turn
                       surrounds the
                       nucleic acid).
                            Some virions are isometric: they have a fully
Virion Attachment Organs     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.
Not-Complex Virions
Complex (Tailed) Phage Virion
       Note that this head actually is elongated top
             to bottom rather than isometric.
Virion Size`   “Big”             “Small”




                                    “Medium”


                       “Even Smaller”
                                             One way to distinguish
                           ssDNA            different types of viruses
                     DNA                   (e.g., influenza virus from
                                             HIV) is in terms of the
Viral Genomes
                           dsDNA
                                             characteristics of their
                                             nucleic-acid genomes.
                Nucleic-Acid Virus Genome

                           dsRNA
                                           Negative
                     RNA
                           ssRNA
                                           Positive

ssRNA  dsDNA  ssRNA
    (the retroviruses)
                                   http://www.virology.net/Big_Virology/BVFamilyGenome.html
Viruses are Not Cells
            “Smaller”   “Bigger”
Some Important Phages
                 Here complex means “tailed”




Note various        An example of
   genome
                      Lysogenic
architectures.       Conversion.
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).
Chronic Infection Strategy
                              This is a
                              process
                               called
                             Extrusion.




                              This is the
                                strategy
                             followed by
                             filamentous
                                phages.
Latent Infection Strategy
                             For phages
                            this is called
                             Lysogeny.



                             Only Temperate
                            phages are able to
                             display lysogeny.
                                     For phages this is
                                     called Lysogenic
                                       Conversion.
Viral Infection Strategies
Virulent Phage Life Cycle
Chronic Phage Life Cycle
Temperate Phage Life Cycle(s)
Temperate Phage Life Cycle(s)                      This is a process
                                                   called Induction.


                                The prophage DNA
                                   can remain
                                    integrated
                                   indefinitely.
                           Adsorption describes the virion‟s attachment process.
Adsorption & Host Range    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.
G. Eliave Institute, Tbilisi, Georgia
G. Eliave Institute, Tbilisi, Georgia
G. Eliave Institute, Tbilisi, Georgia
G. Eliave Institute, Tbilisi, Georgia
G. Eliave Institute, Tbilisi, Georgia
G. Eliave Institute, Tbilisi, Georgia
G. Eliave Institute, Tbilisi, Georgia
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