Bacteriophage are viruses that infect bacteria, characterized by their tadpole shape and dual life cycles: lytic, which leads to the destruction of the host bacterium, and lysogenic, where the phage integrates into the bacterial chromosome. During the lytic cycle, the phage attaches to a bacterium, injects its DNA, and eventually bursts the cell to release new phages, while in the lysogenic cycle, the phage DNA integrates as a prophage and replicates with the bacterium. Bacteriophages are also important for gene transfer between bacteria and have applications in bacterial typing.

1. BacteriophageBacteriophage
Viruses that infect bacteria.
Occur widely in nature in close association
with bacteria.
Readily isolated from faeces, sewage and
other natural sources.

2. MorphologyMorphology
Tadpole shaped, with
hexagonal head and a
cylindrical tail.
Head consists of a tightly
packed core of ds DNA
surrounded by a protein coat or
capsid.
The tail composed of a
contractile sheath surrounding
the hollow core
Terminal base plate having
prongs or tail fibres attached.

3. Life CycleLife Cycle
 Phages exhibit two different types of
life cycle,
1.Virulent or Lytic cycle:
2.Temperate or Lysogenic cycle:

4. Lytic cycleLytic cycle
1. Adsorption:
Phage particles come into contact with bacterial cells by
random collision.
Phage attaches to the surface of a susceptible bacterium
by its tail.
It is a specific process and depends on the presence of
receptor sites on the bacterial cell for the terminal base
plate of the phage.
Cofactors like cations are necessary for adsorption.

5.  2. Penetration :
 The phage DNA is injected into the bacterial body
through the hollow core.
 The infection of a bacterium by the nucleic acid is
known as transfection.
 The base plate and the tail fibres are held firmly
against the bacterial cell.
 The hollow core pierces the cell wall.
 The contractile tail sheath injects the phage DNA .
 After penetration, the empty head and tail of the
phage remain outside the bacterium.

7. 3. Synthesis of Phage components3. Synthesis of Phage components
Synthesis of bacterial protein, DNA and RNA
ceases
First products to be synthesised are called early
proteins which are enzymes.
Subsequently late proteins appear which
include protein subunits of the phage head and
tail.
•

8. 4.4.Assembly & Maturation:Assembly & Maturation:
Phage DNA, head protein and tail
protein are synthesized separately in
the bacterial cell.
• Assembly of the phage components
into the mature infective phage
particles is known
as maturation.

9. 5.Release of Progeny Phage5.Release of Progeny Phage
 Typically occurs by lysis of the bacterial cell.
 Bacterial cell wall is weakened and it assumes
a spherical shape.
 Resulting in the release of mature daughter
phages.

12. Lysogenic CycleLysogenic Cycle
Phages enter into a symbiotic relationship.
Following entry into the host cell, the temperate phage
nucleic acid becomes integrated with the bacterial
chromosome.
The integrated phage nucleic acid is known as the
prophage.
The prophage behaves like segment of the host
chromosome and replicates synchronously with it.

13.  This phenomenon is called lysogeny
 Bacterium that carries a prophage within its genome is
called a Lysogenic bacterium.
 The prophage confers certain new properties on the
Lysogenic bacterium
 This is known as Lysogenic conversion or phage
conversion.
Lysogenic bacterium is resistant to reinfection by the
same or related phages.
This is known as superinfection immunity.

14. Penetration of the Viral Genome into the
Cytoplasm of the Bacterium

15. Functions….Functions….
 Bacteriophages may act as carriers of genes
from one bacterium to another.
 This is known as Transduction
 Eg: Plasmid mediate drug resistance in
staphylococci.
 Eg: Toxin production by the diphtheria
bacillus determined by the presence of beta
phage.

17. Phage typingPhage typing

The specificity of phage-bacterium
interaction is made use of in the
identification and typing of bacteria.

Important application:

For intraspecies typing of bacteria.

Eg: Phage typing of S.typhi and
Staphylococci.