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Viruses of Bacteria
Chapter 13
General Characteristics of Viruses
 Non-living entities
 Not considered
organisms
 Can infect organisms of
every domain

All lifeforms
 Commonly referred to
by organism they infect
General Characteristics of Viruses
 Virus architecture
 Virus particle called virion
 Consists of nucleic acid
surrounded by protein coat
 Protein coat termed capsid
 Capsid composed of capsomers


Virus have different shapes
 Isometric
 Helical
 Complex
Two types of virion
 Naked – without envelope
 Enveloped – surrounded by lipid
membrane
General Characteristics of Viruses
 Viral geneome

Contains only single
type of nucleic acid

 Replication cycle overview

Either DNA or RNA
Only multiply inside
metabolizing cell

 NEVER BOTH


Can be linear or
circular
Single-stranded or
double stranded


Uses host machinery to
support reproduction
Every virus contains
information to make viral
proteins, assure
replication and move in
and out of host cells
Viruses live in two
phases

Extracellular phase
 Metabolically inert

Intracellular phase
 Metabolically active
Virus Interactions with Host Cells
 Effect on cells depends on infecting phage

Some phage multiply inside cell producing
numerous progeny

Termed productive cycle
 Lytic cycle
 Phage lyse infected cell

Some phage integrate into host genome

Termed latent cycle
 Lysogenic state
Virus Interactions with Host Cells
 The six stages of
the lytic cycle are:




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
Adsorption
Penetration
Transcription
Replication
Assembly (or
maturation)
Release
Adsorption (Attachment)
Step 1: Attachment
The phage attach to
specific receptors on
the cell wall of E. coli.
Bacterial DNA
Penetration
 Penetration
 Entrance of the virus OR its nucleic acid in the host
cell


Plant and bacteria viruses inject the nucleic acid into the
host through the cell wall
Animal viruses enter the cell whole
 Animal cells have no rigid cell wall
 Penetration of the virus is through:
 Phagocytosis in which the virus is engulfed by the cell
 Membrane fusion occurs with enveloped viruses when the
viral envelope fuses with the plasma membrane of the host
cell
 Viruses enter intact but require an uncoating step to
release the nucleic acid from the protein coat
Penetration
Step 1: Attachment
The phage attach to
specific receptors on
the cell wall of E. coli.
Bacterial DNA
Step 2: Penetration
Following attachment, phage DNA
is injected into the bacterial cell,
leaving the phage coat outside.
Transcription/Replication
Lytic cycle
 Transcription/Replication


Duplication of viral components
During replication:

Virus will inhibit activity of the host DNA
 Virus produces enzymes to destroy host DNA
 Viral DNA takes over and begins producing proteins
 Early viral proteins are synthesized and are
associated with the replication of viral nucleic acid
 Late viral proteins are synthesized and are
associated with the replication of other viral
structures
Transcription/Replication
Step 1: Attachment
The phage attach to
specific receptors on
the cell wall of E. coli.
Step 2: Penetration
Following attachment, phage DNA
is injected into the bacterial cell,
leaving the phage coat outside.
Bacterial DNA
Step 4: Replication of
Phage DNA and
Synthesis of Proteins
Phage coat proteins,
other protein
components, and
DNA are produced
separately. Host DNA
degraded.
Step 3: Transcription
Phage DNA is transcribed,
producing phage mRNA, which
is translated to phage proteins.
DNA
RNA
Phage-induced
proteins
Assembly
 Assembly (or maturation)

This stage is the assembling of the replicated
viral components into an intact, mature virus
Assembly
Step 1: Attachment
The phage attach to
specific receptors on
the cell wall of E. coli.
Step 2: Penetration
Following attachment, phage DNA
is injected into the bacterial cell,
leaving the phage coat outside.
Bacterial DNA
Step 5: Assembly
Phage components are
assembled into
mature virions.
Step 4: Replication of
Phage DNA and
Synthesis of Proteins
Phage coat proteins,
other protein
components, and
DNA are produced
separately. Host DNA
degraded.
Empty DNA inside
head
head
+
+
+
Step 3: Transcription
Phage DNA is transcribed,
producing phage mRNA, which
is translated to phage proteins.
DNA
RNA
Phage-induced
proteins
Release
 Release

Host cell bursts and releases viruses to the
outside environment


Viruses are now extracellular
As virus leave the host cell the envelope is
picked up

The envelope is made of a portion of the host cell
plasma membrane which becomes the lipid
envelope of the virus
Step 1: Attachment
The phage attach to
specific receptors on
the cell wall of E. coli.
Release
Step 6: Release
The bacterial cell
lyses and releases
many infective phage.
Step 2: Penetration
Following attachment, phage DNA
is injected into the bacterial cell,
leaving the phage coat outside.
Bacterial DNA
Step 5: Assembly
Phage components are
assembled into
mature virions.
Step 4: Replication of
Phage DNA and
Synthesis of Proteins
Phage coat proteins,
other protein
components, and
DNA are produced
separately. Host DNA
degraded.
Empty DNA inside
head
head
+
+
+
Step 3: Transcription
Phage DNA is transcribed,
producing phage mRNA, which
is translated to phage proteins.
DNA
RNA
Phage-induced
proteins
Virus Interactions with Host Cells
 Lysogeny

Replication of a temperate virus


This is a non productive cycle
Lysogeny begins like the lytic cycle

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Adsorption
Penetration, then;
Incorporation
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Virus Interactions with Host Cells
 Incorporation
 Viral nucleic acid incorporates onto the host chromosome
This virus is called a prophage
Once incorporated, repressor genes are expressed and
repressor proteins are produced
 These hide or suppress the viral gene from host
immune responses
The viral DNA replicated only when the host cell replicates
 This allows for a population of bacterial cells that
carry viruses
Cell eventually “pops” off the host chromosome and
returns to the lytic cycle

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
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Virus Interactions with Host Cells
 Lysogenic conversion


Prophage can confer new properties on cell
Phage DNA not completely suppressed

Genes coding for trait are expressed
 Organism displays new trait
 Streptococcus pyogenes manufactures toxin
resulting in scarlet fever due to lysogenic
conversion
Host Ranges of Phages
 Number of different bacteria that phage can
infected termed host range

Usually limited to single bacterial species for a
single phage
 Factors limit host range

Two most important


Phage must be able to attach to host receptors
Restriction-modification system host cell must
overcome
Host Ranges of Phages
 Receptors on bacterial surface

Vary in chemical structure and location


Usually on bacterial cell wall
Sites can be altered by two mechanisms


Receptor sites can be altered by mutation
Lysogenized bacteria can alter cell surface
 Results in alteration of receptor site
Host Ranges of Phages
 Restriction-modification system


Bacteria have two genes
coding for enzymes of
restriction-modification system
Restriction enzyme that codes
for endonuclease

Cuts small segments of
DNA
 May recognize viral DNA
and cut it

Modification enzyme attaches
methyl group to DNA
recognized by restriction
enzyme

Methylated bases not
recognized by restriction
enzyme
 Protects cells own DNA