Bacteriophages, or phages, are viruses that specifically infect and destroy bacteria, initially observed in the early 20th century. They can replicate within bacteria using the host's machinery and exhibit various life cycles, including lytic and lysogenic, with implications for gene transfer and medical applications. Bacteriophages also play a role in enhancing bacterial virulence through lysogenic conversion, carrying genes that can produce toxins and other harmful factors.

1. Bacteriophages
Dr.T.V.Rao MD
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2. 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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4. Bacteriophages under Electron
Microscope
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5. 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

Identification of bacteria - phage typing

Treatment and prophylaxsis???
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6. Bacteriophages:
Definition & History

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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7. 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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8. Bacteriophages:
Classification
 At present, over
5000
bacteriophages
have been studied
by electron
microscopy and
can be divided
into 13 virus
families.
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9. Bacteriophage
 Bacteriophages
make up a
diverse group of
viruses, some of
which have
complex
structures,
including double-
stranded DNA.
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10. Bacteriophage
Bacteriophage
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11. Bacteriophage
Bacteriophage
 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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12. Composition and
Structure

Composition
Composition
− Nucleic acid
Nucleic acid

Genome
Genome
size
size

Modified
Modified
bases
bases
− Protein
Protein

Protection
Protection

Infection
Infection
• Structure (T4)
–
Size
–
Head or
capsid
–
Tail
Tail
Tail Fibers
Base Plate
Head/Capsid
Contractile
Sheath
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14. Phage entering a bacterial cell
Phage entering a bacterial cell
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16. Bacteriophage showing Lytic and
lysogenic cycle
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17. 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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18. Bacteriophages:
Lysogenic Conversion

Some lysogenic phage carry genes that
can enhance the virulence of the
bacterial host.
 For example, some phage carry genes that
For example, some phage carry genes that
encode toxins.
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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19. Bacteriophages
•
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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20. Bacteriophages
•
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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21. Bacteriophages
•
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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22. Lytic and Lysogenic cycle
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25. Bacteriophages:
Lysogenic Conversion
Examples of Virulence Factors Carried by Phage
Bacterium Phage
Gene
Product
Phenotype
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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26. 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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27. Assay for Lytic Phage
•
Plaque assay
–
Method
–
Plaque forming unit
(pfu)
–
Measures infectious
particles
Bacteria
Phage
+
Phage

Plaque assay

Method

Plaque forming
unit (pfu)

Measures
infectious
particles
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28. Lytic
vs
Lysogenic
Cycle
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29. Transduction
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30. Transduction
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31. 31
Figure 13.10.1
Attachment:
Attachment:
Phage
Phage
attaches to
attaches to
host cell.
host cell.
Penetration:
Phage pnetrates
host cell and
injects its DNA.
Merozoites released
into bloodsteam
from liver may infect
new red blood cells
1
2
3
Bacterial
Bacterial
cell wall
cell wall
Bacterial
chromosome
Capsid DNA
Capsid
Sheath
Tail fiber
Base plate
Pin
Cell wall
Tail
Plasma membrane
Sheath contracted
Tail core
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32. 32
Figure 13.10.2
4 Maturation:
Maturation:
Viral components
Viral components
are assembled
are assembled
into virions.
into virions.
Tail
5 Release:
Release:
Host cell lyses
Host cell lyses
and new virions
and new virions
are released.
are released.
DNA
Capsid
Tail fibers
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33. Examples:
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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34. 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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35. 
The Programme Created by Dr.T.V.Rao MD
for Undergraduate Learning in Developing
Countries

Email

doctortvrao@gmail.com
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