Bacteriophages are viruses that infect bacteria, play a crucial role in gene transfer, and are used as models for studying virus-host interactions. Their life cycles include the lytic cycle, where the host bacterium is lysed to release progeny, and the lysogenic cycle, where viral DNA integrates into the bacterial genome without harming the host. Lysogenic bacteria can express new traits due to prophage presence and can be induced to enter the lytic cycle under specific conditions.

1. BACTERIOPHAGE-
LYSIS & LYSOGENY

2. BACTERIOPHAGE
• Viruses that affect bacteria
• Occur widely in nature
• Seen in close association with bacteria
• Readily isolated from feces, sewage, & other natural sources of
mixed bacterial growth
• Convenient model for the study of virus-host interactions
• Play an important role in the transmission of genetic information
between bacteria by the process of transduction
• Presence of phage genome integrated with bacterial
chromosomes confers on bacteria certain properties by PHAGE
CONVERSION

3. • Can be used as a cloning vector in genetic manipulations
• Presence of concentrations upto 10⁸/ml suggest that they
may have a role in the control of bacterial populations

4. MORPHOLOGY
• Infecting E.coli – T phages- T2, T4, T6- served as prototypes in
describing the properties of bacteriophages
• T even phages have a complex morphology
• Tadpole shaped
• Hexagonal head & a cylindrical tail
• Head- Tightly packed core of nucleic acid ( Double stranded DNA)
surrounded by a protein coat or capsid
• Size of head- 28-100nm
• Tail- Hollow core surrounded by contractile sheath & a terminal
base plate which has no prongs, tail fibres or both attached to it
• Spherical or filamentous phages with single stranded DNA or RNA
are also identified

5. Structure of Bacteriophage

6. LIFE CYCLE
• Exhibit different types of life cycle
• Lytic cycle (Virulent cycle)- intracellular multiplication of the
phage culminates in lysis of the host bacterium & the
release of progeny virions
• Lysogenic cycle (Temperate cycle)- Phage DNA becomes
intergrated with the bacterial genome, replicating
synchronously with it causing no harm to host cell

7. LYTIC CYCLE
1. ADSORPTION
• Come in contact with bacterial cell by random
collision
• Attaches to bacterium by tail
• Process depends upon presence of complementary
chemical groups on the receptor sites of bacterial
surface & on terminal base plate of phage
• Bacterial receptor sites are situated on the different
layers of cell wall or on surface structures ( eg- Vi
antigen in typhoid bacillus) or appendages (such as
sex pili or flagella)
• Adsorption is very speedy during optimum conditions
• Cofactors like cation are necessary
• Bacterial protoplast- devoid of cell wall components-
absorb phages
• Level of adsorption determine the host specificity of
phages
• Experimental infection by direct injection of phage
DNA can be achieved even in bacterial strains that are
insusceptible to infection by the whole phage
• Infection of bacterium by the naked phage nucleic
acid is Transfection
2. PENETRATION
• Followed by adsorption
• Resembles injection through a syringe
• Base plate & tail fibres are held firmly against the cell causing the
hollow core to pierce through the cell wall
• Contractile tail sheath acts like muscle & derive its energy from
small amount of ATP present on the tail of the phage
• Phage DNA is injected into the bacterial body through hollow core
• May be facilitated by the presence on the phage tail of lysozyme –
produces a hole on bacterial wall for the entry of the phage core
• Complex structure of phage particle is required for injection of
nucleic acid into host
• Phage DNA alone is necessary for initiation of the synthesis of
daughter phages
• After penetration the empty head & tail of the phage remain
outside bacterium as shell/ghost
• On mixing bacteria with phage particles at high multiplicity –
produce multiple holes- leakage of cell contents- Lysis from
without

8. 3. SYNTHESIS
• Immediately occur after
penetration
• 1st products synthesized –
early proteins – enzymes
necessary for building of
complex molecules peculiar to
phage
• Late proteins appear- include
protein subunits of phage
head & tail
• Synthesis of bacterial protein,
DNA & RNA caeses
4. MATURATION
• Phage DNA, head protein & tail
protein are synthesized separately
in the bacterial cell
• DNA is condensed into a compact
polyhedron & packaged into the
head & finally the tail structures
are added
• Assembly of the phage
components into the mature
infective phage particle is
Maturation

9. 5. RELEASE
 Release of mature progeny
 Occurs by lysis of bacterial cell
 During replication bacterial cell wall is weakened- assumes a spherical shape
 Phage enzyme act on weakened cell wall causing it to burst or lyse- release of
mature daughter phages
 Interval between the entry of phage nucleic acid into bacterial cell & the appearance
of 1st infectious intracellular phage particle- Eclipse phase
 Represent the time required for the synthesis of phage components & their
assembly in mature phage particles

10. • Latent period- Interval between the infection of a bacterial
cell & the 1st release of infectious phage particle
• Immediately after latent period the number of phage
particles released increases for few minutes until maximum
number is attained
• Rise period- Period during which the number of infectious
phages released rises
• Burst size- The average yield of progeny phages per
infected bacterial cell
• Estimated by experiments in which infection is established
with 1 phage/bacterium
• Release of infected phage particles is estimated serially
over a period of time
• Plotted on a graph- One step growth curve

11. One step
growth curve of
Bacteriophage

12. LYSOGENIC CYCLE (Temperate Cycle)
• Temperate phages enter into a symbiotic relationship with their host cell
destroying them
• Following the entry phage nucleic acid becomes integrated with bacterial
chromosome
• Integrated phage nucleic acid- Prophage
• Prophage behaves like a segment of host chromosome & replicates with it -
Lysogeny
• Lysogenic bacterium- Bacterium that carries a prophage within its genome
• Lysogenisation doesn’t upset bacterial metabolism
• Prophage confers new properties on lysogenic bacterium- Lysogenic
conversion/ Phage conversion – due to synthesis of new proteins coded for
by prophage DNA
• Eg:- Toxin production by the presence in it of the prophage beta. Elimination
of prophage abolishes toxicity of the bacillus

13.  During the multiplication of lysogenic bacteria, prophage may be excised from occasional cells
 Excised prophage initiates lytic replication & the daughter phage particles are released & infect other
bacterial cells &render them lysogenic- Spontaneous induction of prophage – rare event
 All lysogenic bacteria in a population can be induced to shift to the lytic cycle by exposure to certain
physical & chemical agents
 Inducing agents- UV rays, Hydrogen peroxide & nitrogen mustard
 Lysogenic bacterium is resistant to reinfection by the same/ related phages – Superinfection immunity