Bacteriophages are viruses that specifically infect bacteria, first described in the early 20th century with significant structural simplicity. They can replicate through two life cycles: the lytic cycle, which destroys the host bacterium, and the lysogenic cycle, which integrates their genetic material into the host's genome without causing harm. Bacteriophages hold potential in medical applications, particularly phage therapy, to combat antibiotic-resistant bacterial infections, although limitations such as the emergence of resistant bacterial strains exist.

1. Bacteriophage

2. Real Image of T4 bacteriophage
via electron microscope

3. Bacteriophage
from the greek phagein, meaning "to eat“
Eaters or destroyers of bacteria
First described in 1915

4. Viruses infecting a bacterium

5. BACTERIOPHAGE
 Viruses that infect and parsitized bacteria
is known as bacteriophage.
It was discovered by Frederick.W.Twort in
Great Britian (1915) and Felix d’ Herelle in
France(1917).
D’ Herelle coined the term bacteriophage
meaning ‘bacterial eater’ to describe the
agent’s bacteriocidal activity. He observed
lysis of a broth culture of a dysentry
bacillus.

6.  Phages are very simple in structure,
consisting merely of a DNA (or occasionally
ribonucleic acid (RNA)) molecule carrying a
number of genes, surrounded by a protective
coat or capsid made up of protein
molecules.
 They can undergo two life cycle
Lytic cycle
Lysogenic cycle

7. Structure of Bacteriophage
•Most of the phages are tadpole-
shaped.
•Phage head: hexagonal in shape,
composed of coat protein and genome in
the core
•Genome: dsDNA codes for enzymes
and proteins necessary to replicate more
viruses
•Tail Sheath: hollow core covered with
a contractile sheath. DNA travels from
head to bacteria through sheath
•Tail fiber: helps anchor the phage on the
cell membrane
•Examples are Lambda phage, M13
phage, T4,T7 phage, P1 phage etc.

8. CHARECTERSTICS OF BACTERIOPHAGES
• Host specificity: they have high host
specificity.
• They are filterable through filters.
• Lytic phage causes lysis of bacteria.
• They are sensitive to heat.
• Their commonest habital is intestinal
bacterial flora of man and animal.

9. PHAGE LIFE CYCLE: LYTIC VS LYSOGENIC
Phage replicates by lytic life cycle
(virulent phage)
•Non-integration of phage genetic material
•Intracellular multiplication of the phage ends in
lysis of host bacterium.
Phage replicates by lysogenic life cycle
(temperate phages (prophages))
•Integration of phage genetic material into
bacterial genome.
•Phage replicates with bacteria causing no harm
to the host cell

10. Adsorption by Lytic Bacteriophage
The bacteriophage binds to specific
receptors on the bacterial cell wall.
Tail conformation changes/contracts
terminal base plate penetrates cell wall

11. An electron micrograph of bacteriophages attached to a bacterial cell. These
viruses are the size and shape of coliphage T1

13. Cycle of events in Bacteriophage infecting
a Bacterial Cell

14. Penetration
• The bacteriophage injects its genome into
the bacterium's cytoplasm

15. Synthesis of phage component
• Phage-coded enzymes shut down host’s DNA, RNA, protein synthesis.
•Intially, early proteins, including specific enzymes, necessary for
synthesis of phage components are synthesized. During this phase, phage
are not detectable also known as eclipse phase.
•Late functions include the subsequent synthesis of other proteins and
assembly of the nucleocapsid.
-MATURATION
After this the protein subunits of phage head, and tail and phage DNA
assembles to form infective phage particle

16. Phage Release
• A bacteriophage-coded enzyme break down the
peptidoglycan in the bacterial cell wall causing osmotic
lysis.

18. Attachment:
Phage attaches
to host cell.
Penetration:
Phage pnetrates
host cell and
injects its DNA.
Biosynthesis:
Transcription/
Translation and
Viral
chromosome
replication
1
2
3
Bacterial
cell wall
Bacterial
chromosome
Capsid DNA
Capsid
Sheath
Tail fiber
Base plate
Pin
Cell wall
Tail
Plasma membrane
Sheath contracted
Tail core
Lytic Lifecycle of a Bacteriophage I

19. 4 Maturation/Assembly:
Viral components are
assembled into
virions.
Tail
5 Release:
Host cell lyses and
new virions are
released.
DNA
Capsid
Tail fibers
Lytic Lifecycle of a Bacteriophage II

20. LYSOGENIC CONVERSION
The prophage confers certain new properties to
lysogenic bacterium.
Examples: of Lysogenic conversion
* 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 cholera 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

21. Bacteriophages Significance
• Phage typing: Used as
epidemiological markers to
identify reservoir of infection.
• Phage therapy: acts as
bacteriocidal agent.
• Acts as a carriers of genes from
one bacterium to another known
as transduction.
• Confers the property of toxin
production in some bacteria.
• Used in studying host- parasite
relationships.

22. TRANSDUCTION
• DNA may be transferred by a
bacteriophage to a bacteria in a
process called transduction.

23. • Phage typing is used as epidemiological marker to
identify different bacterial strains that are
biochemically serologically identical and
indistinguishable from each other.
• Such bacterial strains are differentiated by using
type specific bacteriophage.
• The technique has most extensively been used
for the detection of Mycobacterium tuberculosis,
E.coli, Pseudomonas, Salmonella, Listeria, and
Campylobacter species

24. Bacteriophages in Medicine
• Bacteriophages, or phages, by their
very nature, they can be considered
as potential antibacterial agents.
• Over the past decade or two, the idea
of phage therapy, i.e. the use of lytic
bacteriophages for both the
prophylaxis and the treatment of
bacterial infections, has gained
special significance.
• this leads to the rise in the
prevalence of highly antibiotic-
resistant bacterial strains.

25. Phage Therapy
• Phages were discovered
to be anti-bacterial
agents and were used
throughout the 1940s in
the Soviet Union for
treating bacterial
infections. They had
widespread use including
treating soldiers in the
Red Army. However, they
were abandoned for
general use in the west.
25

26. S.aureus infection Treated with phage
impregnated pad
Improvement in wound healing

27. Limitations of phage therapy
1.Emergence of bacterial strains resistant to
particular phages. The emergence of phage –
resistant bacterial mutants was observed and
the phenomenon was suggested to be a
potential problem of phage therapy
(Summers, 1999; d’Herelle,1930)