Viruses that infect bacteria.
Occur widely in nature in close association with bacteria.
Readily isolated from faeces, sewage and other natural sources.
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.
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.
6.
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.
10.
11.
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.
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.
16.
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.