Bacteriophages are viruses that infect and replicate inside bacteria. They were first observed in 1917 by Twort and d'Herelle who noticed that bacterial cultures could be dissolved by adding filtrate from sewage. Bacteriophages have complex structures when viewed under electron microscopes and there are over 5000 types classified into 13 families. They carry only the genetic information needed for replication and use the host cell's machinery. Bacteriophages can induce lysogenic conversion by integrating their DNA into the host chromosome, which may enhance the host's virulence by allowing it to produce toxins. They have potential medical applications as alternatives to antibiotics.

1. Bacteriophages
1Presented by EKTA

2. What are Bacteriophages
Viruses that attack bacteria were observed by Twort
and d'Herelle in 1917. They observed that broth
cultures of certain intestinal bacteria could be
dissolved by addition of a bacteria-free filtrate
obtained from sewage

3. 3

4. Bacteriophages under Electron
Microscope
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5. 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???

6. Bacteriophages:
Definition & History

6

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.

7. 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.

8. Bacteriophages:
Classification
 At present,over
5000
bacteriophages have
been studied by
electron microscopy
and can be divided
into 13 virus
families.
8

9. Bacteriophage
Bacteriophages
make up a diverse
group of viruses,
some of which
have complex
structures, including
double- stranded
DNA.
9

10. MORPHOLOGY
 Bacteriophage are basically tadpole-shaped
possessing a hexagonal head and a tail attached
with tail fibers.
1- head
2- tail

11. Bacteriophage
. . 11

12. Bacteriophage

. . 12
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.

13. . . 13

14. Phage entering a bacterial cell
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15. 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.
. . 15

16. Life cycle

17. Bacteriophages:
Lysogenic Conversion

Some lysogenic phage carry genes that
can enhance the virulence of the
bacterial host.


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.
. . 17

18. 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
. . 18

19. 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
. . 19

20. Lytic and Lysogenic cycle
. . 20

21. . . 21

22. Bacteriophages:
Lysogenic Conversion
Examples of Virulence Factors Carried by Phage
. . 22
Bacterium Phage Phenotype
Vibrio cholerae
Gene
Product
cholerae toxin
Escherichia coli shigalike toxin
Clostridium botulinum
cholera
hemorrhagic
diarrhea
botulism (food
poisoning)
CTX phage
lambda
phage
clostridial
phages
corynephage
beta diphtheria
Corynebacterium
diphtheriae
Streptococcus
pyogenes
T12
botulinum
toxin
diphtheria
toxin
erythrogenic
toxins scarlet fever

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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.

24. Assay for Lytic Phage
•
–
Phage
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Bacteria
+
Phage
 Plaque assay
 –
Method
 Plaque forming
–unit (pfu)
 Measures
infectious
particles

25. Transduction
. . 25

26. 3
1
Penetration:
Phage pnetrates
host cell and
injects its DNA.
Merozoites released
into bloodsteam
from liver may infect
new red blood cells
1 Attachment:
Phage
attaches to
host cell.
2
3
Bacterial
cell wall
Bacterial
chromosome
Capsid
Ca
S
T
Ba
Pin
Ce
P
Sh
Ta
. .

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4 Maturation:
Viral components
are assembled
into virions.
5 Release:
Host cell lyses
and new virions
are released.
Tail
DNA
Capsid
Tail fibers
. .

28. 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
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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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