Power point presentation for under graduate medical students

1. Bacteriophage
Dr. Pendru Raghunath Reddy
Assistant Professor
Dept of Microbiology
Dr. VRK Women’s Medical College

2. Bacteriophage
Definition:
Bacteriophages are viruses that infect and parasitise bacteria
d’Herelle named the bacteriophage (Greek phagein, to eat)
Phages occur widely in the environment such as sewage, faeces, soil
and other natural sources of mixed bacterial growth

3. 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 head and a tail
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

4. 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

5. Life cycle
Phages exhibit two different types of life cycle
1. Virulent or lytic cycle
2. Temperate or lysogenic cycle
Lytic cycle
Replication of a virulent phage can be divided into five stages
a) Adsorption
b) Penetration
c) Synthesis of phage components
d) Maturation
e) Release

6. Adsorption
 Phage attaches to a specific receptor site by means of tail fibres
 Adsorption is a specific process and depends on the presence of
complementary chemical groups on the receptor sites and on the
terminal base plate of the phage
 Infection cannot occur in the absence of adsorption
 The infection of a bacterium by the naked phage nucleic acid is
known as ‘transfection’

7. Penetration

8. Virulent or lytic cycle of bacteriophage
Eclipse phase
The interval between the entry of the phage nucleic acid into the bacterial
cell and the appearance of first infectious intracellular phage particle

9. Lysogenic cycle
 In lysogenic cycle, the bacteriophage nucleic acid becomes inserted
into the bacterial chromosome. The integrated phage genome in this
state is known as the ‘prophage’
 The prophage behaves like a segment of the host chromosome and
replicates synchronously with it. This phenomenon is called ‘lysogeny’
 The bacterium that carries a prophage is known as a
‘lysogenic bacterium’
 The prophage confers certain new properties on the lysogenic
bacterium. This is known as ‘lysogenic conversion or
phage conversion’
 Bacteriophage that parasitise a bacterium without lysing it is known as
‘temperate phage’

10. Bacteriophages:
Lysogenic Conversion
Virulence factors coded by prophage
Gene Medical
Bacterium Phage
Product importance
Vibrio cholerae CTX phage cholerae toxin cholera
lambda hemorrhagic
Escherichia coli shigalike toxin
phage diarrhea
CEβ and botulinum botulism (food
Clostridium botulinum
DEβ toxin poisoning)
Corynebacterium diphtheria
beta phage diphtheria
diphtheriae toxin
Streptococcus erythrogenic
T12 scarlet fever
pyogenes toxins

11. Events Leading to Lysogeny
 Circularization of the phage chromosome
 Cohesive ends
Cohesive Ends
Lygase
Linear Double Stranded Opened Circle Closed Circle

12. Events Leading to Lysogeny
 Site-specific
recombination
 Phage coded enzyme
gal bio
 Repression of the
phage genome
 Repressor protein gal
bio
 Immunity to
superinfection
gal bio

13. Termination of Lysogeny
 Occasionally integrated
prophage may become gal bio
excised from bacterial DNA
and initiates lytic cycle and gal bio
the daughter phage particles
are released, which infect
other bacteria and render
them lysogenic gal
bio
 This is known as ‘spontaneous
induction of prophage’
gal bio

14. Significance of phages
1. Virulent phage
a) Phage typing
b) Phage therapy
c) Phage display
2. Temperate phage
a) Toxin production b) Antigenic property
c) Transduction d) Cloning vector

17. Phage display