This document discusses bacteriophages, which are viruses that infect bacteria. It describes their morphology, including their hexagonal heads and tails. It also outlines their two life cycles: the lytic cycle, which results in the bacterial cell lysing and releasing new phages, and the lysogenic cycle, where the phage DNA integrates with the bacterial genome without killing the host cell. The document notes that bacteriophages have significance for uses such as phage therapy to treat bacterial infections and phage typing to identify pathogenic bacteria.

1. BACTERIOPHAGE
By Monika Kafaliya
BSc Medical
Microbiology

2. • INTRODUCTION
• MORPHOLOGY
• LIFE CYCLE
- Lytic Cycle
- Lysogenic Cycle
• SIGNIFICANCE /USES
OF BACTERIOPHAGE
INDEX

3. INTRODUCTION
• Bacteriophages are viruses that infect bacteria
• Twort (1915) – described degenerative change in
Staphylococcal colonies
• d'Herelle (1917) – observed that filtrates from feces
culture from dysentery patients induced transmissible
lysis
• He suggested that lytic agent was a virus and named
it Bacteriophage.

4. INTRODUCTION [contd. ]
• Phages occur widely in nature in close association with
bacteria
• Can be readily isolated from feces and sewage and
other sources of mixed bacterial growth
• Viruses are small, acellular particles that can replicate
only in a host cell.
• They are obligatory intracellular parasites. They consist
of a nucleic acid genome enclosed in a protective
protein shell or capsid.

5. MORPHOLOGY
• Bacteriophages that infect E.
coli, called the T-even phages
(T2, T4, T6),have been
extensively studied.
• T-even phages are tadpole-
shaped, possess a hexagonal
head and a tail attached with
tail fibers.

6. 1. Head -
Hexagonal in shape and
consists of a tightly packed
core of nucleic acid (double
stranded DNA) enclosed by a
protein coat, called a
capsid .
The size of the head varies in
different phages from 28 nm
to 100 nm.

7. 2. Tail -
It is composed of a hollow
core surrounded by a
contractile sheath, and a
terminal base plate which has
attached to prongs, tail fibres
or both Head/Capsid
Contractile Tail Sheath Tail
Fibers Base Plate.

8. LIFE CYCLE
Bacteriophages exhibit two different types of life cycles -
1. Virulent or Lytic Cycle :- are phages which can only
multiply in bacteria and kill the cell by lysis at the end of the
life cycle.
2. Temperate or Lysogenic Cycle :- are the phage DNA either
becomes integrated with the bacterial genome or exists as a
free plasmid in the bacterial cell and replicates
synchronously with it causing no harm to the host cell.

9. 1. Lytic Cycle(seen in Virulent
Phages)
Replication of a virulent phage can be divided into
five stages :-
1. Adsorption
2. Penetration
3. Biosynthesis
4. Maturation
5. Release

10. 1. Adsorption :- [Phage attaches to Host cell surface ]
• Phage attaches to a specific receptor site by means of
tail fibres
• The phages come in contact with the bacterial cells
by random collision and attach to specific receptors
on bacterial cell by means of tail fibre.
• This is essential to initiate the life cycle.
• Infection cannot occur in the absence of
adsorption
• The infection of a bacterium by the naked phage
nucleic acid without capsid is known as
‘transfection'.

11. Adsorption

12. 2. Penetration :- [Phage penetrates host cell and
injects its DNA]
• Phage nucleic acid penetrates into bacterial cell .
• Resembles injection through syringe
• Penetration facilitated by lysozyme producing hole in
bacterial cell wall
• After penetration, empty head and tail of phage: shell
or ghost on bacterial cell

13. Penetration

15. 3. Biosynthesis :- [Phage DNA directs synthesis of viral
components by the host cell]
 The phage DNA directs production of phage proteins
and copies of the phage genome by host and viral
enzymes, using components within the cell.
• Early proteins: enzymes necessary for building complex
molecules
• Late proteins: including protein subunits of phage head and
tail
• Bacterial metabolism remain inhibited during the entire
process

16. Biological synthesis

17. 4. Assembly & Maturation :- [Viral Components are
assembled]
• Phage DNA, head protein and tail protein: synthesised
separately.
• DNA condensed and ‘packaged’ into head.
• Finally tail structures are added.
• This assembly of phage components into mature infective
phage particle is known as Maturation.
5. Release :- [Host cell lyses and new virions are released]
• Phage enzymes cause bacterial cell wall to burst or lyse.
• Daughter phages released.

18. Maturation and Release

21. 2. Lysogenic Cycle (seen in Temperate
Phages)
 Lysogenic or temperate phages are those that can
either multiply via the lytic cycle or enter a quiescent
state in the cell.
 The phage DNA in this repressed state is called a
prophage (Before eating) because it is not a phage but
it has the potential to produce phage.
 Lysogenic Cycle allows replication of phage genome
without destroying the host.

22. Life cycle of Lysogenic or Temperate phase
 Adsorption
 Penetration
 Prophage Formation
 Binary Fission (Replication of bacterial cell)

23.  Adsorption Or Attachments

24.  Penetration

25.  Prophage Formation
Attachmnent of Viral DNA to the
Bacterial DNA.

26.  Binary Fission

27. Application and Uses
 Used in treatment of bacterial
infections (Phage Therapy)
 Used for the identification of
pathogenic bacteria (phage typing)
 Used in molecular biology

28. THANK YOU
MA'AM
😊