LECTURE 3:
Life Cycle of
Bacteriophages
Viro102:
Bacteriophages & Phage Therapy
3 Credit hours
NUST Centre of Virology & Immunology
Life Cycle of Bacteriophages
 Consists of the
following steps
1.
2.
3.
4.
5.
6.
Adsorption
Penetration
Replication
Maturation
Release
Reinfection
1. Adsorption
Bacteriophages attach to specific receptors on
bacterial cell through tail fibers.
Receptors include:
o LPS, Proteins, Teichoic acids, Flagella, Pilli.
Examples:
o lambda phage (λ) infects E. coli K12 by J protein
(tail fiber) attachment to lamB gene product (host cell
wall).
o T4 Phage attaches to LPS on E. coli O157:H7 (host
cell wall) through tail fibers.
o Phage Cbk binds to pilli on Caulobacter
crescentus.
A-1
Coliphage T-1
attached to bacterial
cell wall
Cont’d
Tail fibers & host cell recptors linked by
weak interactions & this association is
reversible.
Once the host has been recognized, more
tail fibers make contact & the base plate
settles down.
Base plate converts reversible attachment
into irreversible
2. Penetration
Irreversible attachment induces conformational
changes in the contractile sheath & tail,
causing the genome to move into the hollow
tube.
Entry hole for genome in cell wall made by
o Enzyme released by the virus (lysozyme).
o Or Contractile sheath, itself.
Only the viral genome enters the host cell
leaving the protein coat of the phage outside.
3. Replication
Once Inside the cell, a bacteriophage
decides between:
o Lytic life cycle: Intracellular replication of phage
followed by progeny release.
o Lysogenic life cycle: Phage genome integrates into
the bacterial genome or exists as a free plasmid in the
host cell replicating with it without harming it.
Genome Organization
Early genes
Expression seen in
within 2 min after
infection
Helps phage in taking
over host cell
E.g. modification of
sigma (σ) factor of Host
RNA polymerase
Middle genes
5 min after
infection
Products of these
genes help in the
replication of the
phage genome
Late genes
Direct the
synthesis of
capsid proteins &
assembly of
phage
Decision between LYTIC or
LYSOGENIC cycle
Repressor Protein
LYSOGENIC CYCLE
Activator
Protein
LYTIC CYCLE
LYSOGENIC CYCLE
1. Temperate Phage (High
repressor protein)
Lysogenic Life Cycle
The phage that can undergo lysogenic life
cycle is Temperate phage.
The phage inside the cell becomes
inactivated that is it does not express its
genes
When the bacterial cell divides into two,
the phage is transferred into both daughter
cells
Cont’d
This sort of relationship between the host
bacterial cell and the virus is called
Lysogeny
o Phage = Prophage
o Host cell = Lysogen
Surface proteins of the host are changed
so there is no super infection. This is
known as Super infection immunity.
Cont’d
Prophage can exist in 2 forms
o Integrated into the host genome.
o Circularized form.
Temperate phages have a gene int that
encodes an enzyme “integrase”. This is
responsible for integration of phage
genome into host genome.
Integration through Site specific
recombination.
Site Specific Recombination
Cont’d
Hostile conditions (UV exposure, desiccation
ionizing radiation etc) can cause excision of the
viral genome and termination of lysogeny, a
process called induction. Lytic life cycle starts
after induction of prophage.
When a cell becomes lysogenized, occasionally
genes carried by the phage get expressed in the
cell. These genes can change the properties of
the bacterial cell. This process is known as
lysogenic conversion or phage conversion.
4. Maturation
Early genes
Middle genes
Late genes
1. Phage structural proteins
2. Proteins that help in phage assembly without
becoming part of phage
3. Proteins involved in cell lysis
5. Release
Assembled phages directs production of an
enzyme that breaks down the bacteria cell wall
and allows fluid to enter.
Enzymes may include:
o Endolysin
o Holin
This results in cell bursting or lysis; The new
viruses are then free to infect other cells
Re-infection
 An average of 50 to 200 phages may be
produced per infected bacterium (max 1000)
 Bacteria always exist in the form of colonies.
 Phages released from one cell can easily
penetrate another cell in the surrounding.
 In labs, plaque observations are done based on
this same principle.
The average yield of phages per infected
bacterial cell is known as burst size.
Bacterial Defense System
Possibility of mutations of cell surface
receptors
Restriction enzymes & RE site
o Cut the viral DNA
o Protect themselves by methylation of their
own genome
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