Bacteriophages
Dr.T.V.Rao MD
4/13/2015
Dr.T.V.Rao MD's Undergraduate Series
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What are Bacteriophages
Viruses that attack bacteria were
observed by Twort and d'Herelle in
1915 and 1917. They observed that
broth cultures of certain intestinal
bacteria could be dissolved by addition
of a bacteria-free filtrate obtained from
sewage
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Bacteriophages under Electron
Microscope
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Bacteriophage (Phage)


Definition - Obligate intracellular parasites
that multiply inside bacteria by making use of
some or all of the host biosynthetic
machinery
Significance

Models for animal cell viruses

Gene transfer in bacteria

Medical applications
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
Identification of bacteria - phage typing

Treatment and prophylaxsis???
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Bacteriophages:
Definition & History
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Bacteriophages are
viruses that can infect
and destroy bacteria.
They have been
referred to as bacterial
parasites, with each
phage type depending
on a single strain of
bacteria to act as host.
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BACTRIOPHAGES
Like most viruses, bacteriophages
typically carry only the genetic
information needed for replication of
their nucleic acid and synthesis of their
protein coats.. They require precursors,
energy generation and ribosomes
supplied by their bacterial host cell.
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Bacteriophages:
Classification
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At present, over
5000
bacteriophages
have been studied
by electron
microscopy and
can be divided
into 13 virus
families.
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Bacteriophage
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Bacteriophages
make up a
diverse group of
viruses, some of
which have
complex
structures,
including doublestranded DNA.
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Bacteriophage
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Bacteriophage
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Also known simply as a
phage; a virus that
attacks and infects
bacteria. The infection
may or may not lead to
the death of the
bacterium, depending on
the phage and
sometimes on
conditions. Each
bacteriophage is specific
to one form of bacteria.
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Composition and
Structure
Composition

 Nucleic
acid
Genome
size
 Modified
bases
Head/Capsid

•
Structure
Protein
(T4)
–
–
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–
Contractile
Sheath
Tail Fibers
Size
 Protection
Head or
 Infection
capsid
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Tail
Tail
Base Plate
12
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Phage entering a bacterial cell
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Bacteriophage showing Lytic and
lysogenic cycle
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Bacteriophages:
Virulence Factors Carried On Phage

Temperate phage can go through one of two
life cycles upon entering a host cell.
1)
Lytic:
Is when growth results in lysis of the host and release of
progeny phage.
2)
Lysogenic:
Is when growth results in integration of the phage DNA
into the host chromosome or stable replication as a
plasmid.
Most of the gene products of the lysogenic phage
remains dormant until it is induced to enter the lytic
cycle.
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Bacteriophages:
Lysogenic Conversion

Some lysogenic phage carry genes that
can enhance the virulence of the
bacterial host.
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For example, some phage carry genes that
encode toxins.
These genes, once integrated into the
bacterial chromosome, can cause the
once harmless bacteria to release
potent toxins that can cause disease.
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Bacteriophages
•
•
•
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Used for cloning foreign
genes among other
applications
Proteins and peptides are
fused to the
Capsid(surface) of the
phage
The combination of the
phage and peptide is
known as a Fusion Protein
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Bacteriophages
•
•
•
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Used for cloning foreign
genes among other
applications
Proteins and peptides are
fused to the
Capsid(surface) of the
phage
The combination of the
phage and peptide is
known as a Fusion Protein
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Bacteriophages
•
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Once these Phages are isolated and
recovered they can be used to infect
bacteria which will create a particle similar
to a monoclonal antibody
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Lytic and Lysogenic cycle
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Bacteriophages:
Lysogenic Conversion
Examples of Virulence Factors Carried by
Phage
Gene
Bacterium
Phage
Vibrio cholerae
CTX phage
cholerae toxin
cholera
Escherichia coli
lambda
phage
shigalike toxin
hemorrhagic
diarrhea
Clostridium botulinum
clostridial
phages
botulinum
toxin
botulism (food
poisoning)
Corynebacterium
diphtheriae
corynephage
beta
diphtheria
toxin
diphtheria
Streptococcus
pyogenes
T12
erythrogenic
toxins
scarlet fever
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Product
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Phenotype
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Lysogenic conversion
In some interactions between lysogenic
phages and bacteria, lysogenic conversion
may occur. It is when a temperate phage
induces a change in the phenotype of the
bacteria infected that is not part of a usual
phage cycle. Changes can often involve the
external membrane of the cell by making it
impervious to other phages or even by
increasing the pathogenic capability of the
bacteria for a host.
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Assay for Lytic Phage
Phage
•
Plaque assay
Method
Method
Plaque forming unit
Plaque
forming
(pfu)
unit
(pfu) infectious
Measures
particles
Measures
infectious
particles

Plaque assay

–
–

Bacteria
+
Phage
–

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Lytic
vs
Lysogenic
Cycle
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Transduction
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Transduction
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Bacterial
cell wall
Bacterial
chromosome
Capsid
DNA
Capsid
Sheath
Tail fiber
1 Attachment:
Base plate
Pin
Cell wall
Phage
attaches to
host cell.
Tail
Plasma membrane
2
Penetration:
Phage pnetrates
host cell and
injects its DNA.
Sheath contracted
Tail core
3 Merozoites released
into bloodsteam
from liver may infect
new red blood cells
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Figure 13.10.1
Tail
DNA
4
5
Maturation:
Viral components
are assembled
into virions.
Capsid
Release:
Host cell lyses
and new virions
are released.
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Tail fibers
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Figure 13.10.2
Examples:
* Corynebacterium diphtheria produces the
toxin of diphtheria only when it is infected by the
phage β. In this case, the gene that codes for the
toxin is carried by the phage, not the bacteria.
* Vibrio cholerae is a non-toxic strain that can
become toxic, producing cholera toxin, when it is
infected with the phage CTXφ.
* Clostridium botulinum causes botulism.
* Streptococcus pyogenes causes scarlet fever.
* Shiga toxin
Tetanus
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Undergraduate Series
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Medical Applications of Phages
“I strongly believe phage could become
an effective antibacterial tool” - Carl Merril,
Chief of the Laboratory of Biochemical Genetics,
National Institute of Mental Health, NIH.

“It might be another string on the bow,
such that when (conventional
antibiotics) fail, here’s something that
has a chance of working. But it’s not
going to be a panacea” - Joshua Lederberg,
Sackler Foundation Scholar at The Rockefeller
University
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
The Programme Created by Dr.T.V.Rao MD
for Undergraduate Learning in Developing
Countries


4/13/2015
Email
doctortvrao@gmail.com
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