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 capsidBacteriophage is the virus that infect bacteria.Bacteriophages were discovered by Frederick Twort(1915)and Felix d'Herelle(1917).

1. BACTERIOPHAGES
By:-
Feba Shaji
MSc Biochemistry

2. INTRODUCTION
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.

3. A BACTERIOPHAGE
Bacteriophage is the
virus that infect
bacteria.
Bacteriophages were
discovered by Frederick
Twort(1915)and Felix
d'Herelle(1917).

4. Bacteriophages have nucleic acid
surrounded by capsid.
The capsid is made up of capsomeres.
Capsomeres consist of protein subunit
known as protomeres.

5. COLIPHAGES
Coliphages were discovered by Herelle.
Bacteriophages attacking E.coli are called
as Coliphages and are designated T-type.
These were numbered T1,T2,T3,.....T17
by Max Delbruck(1938).
T2,T4,T6... are known as T-even phages.
Rest are called as T-odd phages.

6. CYANOPHAGES
Cyanophages were discovered by
Schafferman and Morris(1963).
First cyanophage was discovered from
three blue green algal genera ie,
Lyngbya,Phormedium and Plectonema,
hence also known as LPP-1.

8. STRUCTURE OF A T4 PHAGE
• It contains a head and a tail region.
• Head is icosahedron with about 2000
capsomeres and contain a 53 micrometer long
double stranded DNA in it.
• Tail is helical symmetry with a core tube(80
angstrom in diameter) surrounded by a protein
sheath (consist of 144 subunits arranged in 24
ruins of six subunit each).
• Sheath is connected to collar at its upper end
and a base plate at its lower end.

9. • Base plate is hexagonal with single pin or peg
or spike at each corner.
• A thin tall fiber(1300A long) is also given out
from each corner of base plate.
• Head is prism like hexagonal having
950X650A° dimension.
• Tail is 950A° in length joined to head by neck
and collar.

11. Bacteriophages may be classified into
different morphological class
• A - These are the most complex have
hexagonal head and a rigid tail with a
contractile sheath and tail fibres.
• Example - Coliphages T2,T4,T6
• B - These are similar to A but lack contractile
sheath on the tail and may or may not have
tail fibres.
• Example - Coliphages T1,T5

12. • C - These are same as B type but their tail is
shorter than head.
• Example - Coliphages T3,T7.
• D - These have head made up of large
capsomeres but no tail.
• Example - ¢X 174 ,S13.
• E - These are similar to D type instead they
have small capsomeres.
• Example - f2 , MS2.

13. • F - These are filamentous bacteriophage.
• Example - Coliphages fd , f1.
• G - These have lipid envelope and are
polymorphic.
• Example - MV - L2

18. STEPS INVOLVED IN THE INFECTION
OF BACTERIOPHAGE
• ADSORPTION
• PENETRATION OR INJECTION
• ECLIPSE STAGE
• MATURATION OR VIRUS ASSEMBLY
• LYSIS AND RELEASE OF NEW VIRUSES

19. ADSORPTION
Attachment of phage to the bacteria is known as
adsorption.
This occurs when a phage collides with the host
bacteria.
Phage adsorbs to specific receptors(F pili,
lipoproteins,iron transport protein etc.) on the
bacteria by the tip of the tail.
Example : T4 and T7 Coliphages bind to lipoprotein
receptors.

21. PENETRATION OR INJECTION
• The tail fibres of the virus bend to bring the
spikes and basal plate in contact with the
surface of bacterial wall.
• The sheath is pressed and the viral genome
injected to the bacterial cytoplasm.
• The empty capsid that remains out is called
as the ghost or doughnut.
• This step is active and requires ATP.

23. THE ECLIPSE STAGE
This stage shows following activity in the host
cell : -
• Immunity against infection by same phages
by the production of specific enzyme called
as repressors.
• Suppression of all cellular activity of host.
• Synthesis of proteins coded by phage DNA
using hosts amino acid pool. The protein
synthesized is called as early proteins.

24. • These proteins are enzymes and are used to
seal the hole in cell wall, to destruct the DNA
of host.
• The fresh DNA molecules of phage synthesize
late protein which are the viral coat proteins
and viral lysozymes.
• The coat protein are monomer which are
assembled into capsid and other viral
components.

26. MATURATION OR VIRUS ASSEMBLY
• Head and tail are first assembled separately
then they are attached to form viral particles.
• The time period between injection of viral
nucleic acid and the first appearance of
phage is known as eclipse period.
• Time period between injection of viral
nucleic acid to rupture of host cell is known
as latent period.

28. LYSIS AND RELEASE OF NEW VIRUSES
• The cell wall burst out at the end of latent
period and the phages are liberated .
• This phenomenon is known as lysis.
• The number of virions produced per host cell
is specific and is termed as burst size.
• Generally it is 200 - 300.

30. What are phages good for?
• They play important role in ecology and
evolution of bacteria.
• Phages assist identification, classification and
detection of pathogenic bacteria.
• They are used in phage therapy(in order to
specifically destroy pathogens).
• They eliminate pathogens like Campylobacter
and Listeria in fresh food to reduce food
spoilage.

31. THANK
YOU