Bacteriophage, or phages, are viruses that infect and replicate within bacteria. They were discovered in the early 1900s and are classified based on their nucleic acid and structure. Phages have either a lytic or lysogenic lifecycle - the lytic cycle kills the host bacteria to release new phages, while the lysogenic cycle integrates the phage DNA into the host genome. Phages have potential applications in medicine, biotechnology, and agriculture due to their ability to target specific bacteria.

1. Bacteriophage

2. Introduction:
- A virus that infects
- Replicates within bacterium
- Derived from “bacteria” and phagein, “to
devour-to eat”
- Single or double stranded
- Surrounded by protein capsid
- Genomes may codes as few as four genes and as
many as hundred of genes.

3. Bacteriophages
- Bacteriophages were discovered independently
by Twort 1915 in U.K. and Herelle 1917 in
France and the term was coined by Herelle.
- Bacteriophage or Phage is a virus that infects
and replicates only within the body of bacteria
and no other organism.
- Highly species-specific towards their host cell.
The bacteriophage only infect a single species of
bacteria or even specific strains of bacteria
within a species.

4. Bacteriophage:
- Have a potential to destroy bacterial suspension
and called Twort d Herelle Phenomenon.
- Bacteriophage attacking Escherichia coli are
called coliphages or T phages.
- Like all viruses, phages are simple entities (DNA
or RNA) that consist of a core of genetic material
(nucleic acid) surrounded by protein capsid.

5. - The nucleic acid may be either DNA or RNA
and may be double stranded or single stranded.
- The international committee on Taxonomy of
viruses had classified the phages based on the
nature of their nucleic acid, morphology and
physiochemical properties of their virion
particles. Currently ICTV has classified the
bacteriophages into 9 orders, 48 families with
32 subfamilies and 317 genera.

6. - By morphology phages can be tailed phages,
polyhedral (flat sides usually more than 6) or
filamentous phages.

7. Structure (T Even Phage)
- The virion of T even phage is tadpole like structure
with a hexagonal (six side and six angle) head
connected to a helical tail through a short collar.
- The head is about 95 nm in length and 65 nm in
diameter and composed of about 2000 capsomeres
and encloses a tightly packed nucleic acid.
- The linear dsDNA (50nm long)is coiled,very long and
it is genetic material of phage particles and includes
the host cell to synthesize more and more phage
particle.

8. - The tail has an inner hollow tube called core,
surrounded by a contractile sheath which consist
of 24 annular rings. The distal end of the tube is
connected to a hexagonal basal plate with spike
or tail spin at each corner. Six long, flexible tail
fibers (150nm) arise from basal plate which helps
the bacteriophage to attaches to the bacteria.

10. Lytic Cycle:
Depending upon the interaction of phages with
the bacterial cells they have been distinguished
into two major types of life cycle:
a)Lytic cycle or virulent cycle or infective cycle
(Virulent phage): Type of lifecycle where the
viral DNA remains as a free floating molecule
and replicates separately from the bacterial
DNA.
b)Lysogenic cycle or Temperate Cycle (Non or
Avirulent Phage): is defined by the incorporation
of the bacteriophage genome into the host
genome.

11. a)Lytic Cycle:
- In the lytic cycle, a bacteriophage infects a
bacteria and kills it to release progeny virus.
There are five stages in the bacteriophage lytic
cycle.
i)Attachment: The bacteriophage attaches itself on
the surface of bacteria (adsorption). The tips of
the tail fibres attach to specific receptors eg:
lipopolysaccharides, OmpC protein on host
surface (bacterial cell).

12. ii)Penetration:
Lysozyme digest the host cell wall and the tail
sheath contracts, which act like a hypodemic
needle to inject the viral genome (DNA) in the
host cell. The phage head and remaining
components remain outside the bacteria called
Ghosts or Doughnut.

13. iii) Biosynthesis of new viral component: After
entering the host cell, the virus synthesizes virus-
encoded endonucleases to degrade the bacterial
chromosome. It then hijacks the host cell to
replicate,transcribe and translate the necessary
viral component (capsomeres,sheath,base
plate,tail fibers and viral enzymes) for the
assembly of new viruses.

14. iv) Maturation and assembly: On maturation, the
head and tail protein of phage DNA assemble
and each component of phage DNA is
surrounded by a protein coat. Ultimately, the tail
structure are added forming a virion.
v) Release: The infected bacterial cell is lysed
releasing the progeny phages and progeny
viruses are liberated into the environment to
infect the cells. The phage enzymes weaken the
cell wall of bacteria during replication.

16. Lysogenic cycle:
- In a lysogenic cycle, the phage genome enters the cell
through attachment and penetration. A prime
example is the lambda phage(a non-contractile tailed
phage, meaning during an infection event it cannot
'force' its DNA through a bacterial cell membrane.)
- During the lysogenic cycle, instead of killing the
host, the phage DNA integrates into bacterial
chromosome and becomes part of the host. The
integrated phage genome (DNA) is called a
prophage.
- A bacteria carrying a prophage without being lysed is
called a lysogen and the process is called lysogeny.

17. - As the bacteria replicate its chromosome, it also
replicate the phage’s DNA and passes it on to
new daughter cells during reproduction.
- The presence of the phage may alter the
phenotype to the bacterium,since it can bring in
extra genes. This change in the host phenotype is
called lysogen conversion or phage conversion.
In the case of V.cholera phage encoded toxin can
cause severe diarrhea and in C.botulinum, the
toxin can cause paralysis and both are less
virulent in absence of the prophage.

18. During lysogeny, when the host cell face harsh
condition then the prophage shows induction,
which results in the excision of the viral genome
from the host chromosome. After induction the
temperate phage follows the lytic cycle then again
undergoes lysogeny in a newly infected cell.
The lytic cycle doesnot allow genetic
recombination of the host chromosome
whereas the lysogenic cycle allow the genetic
recombination of host chromosome.

20. Economic Importance:
Advantage
- Phages are very specific and do not harm the
useful bacteria that live in and out of body.
- They have been proposed as alternative to
antibiotics resistance bacterial strains.
- Have been used in treatment against dysentery,
cholrea, plague and many other pathogenic
bacterial disease.

21. - Useful in the lysis of bacterial present in polluted
water.
- Helps in transduction of genetic material and
therefore they have potential uses in
biotechnology, research and therapeutics.
- Phages can be used as bio control agents in
agriculture and petroleum industry.
- Phages can be administered orally, incorporated
in drinking water or food to control Salmonella
and Campylobacter in poultry.
- Phages are also excellent as food bio
preservation agents.

22. Disadvantages:
- In comparison to chemical molecules, phages are
larger. So, the sites in the body that can be
reached by them must be carefully clarified.
- Phages that are injected into bloodstream are
recognized by the human immune system. Some
of them are quickly excreted and after a certain
period antibodies against the phages are
produced by the body. For example: it appear
that one type of phage can only be used for
intravenous treatment.

23. Diseases caused by Bacteriophages:
- Bacteriophages only infect bacteria, they do not
cause disease in human.
- Alter the genome of non-virulent bacteria
strains, thus, producing more virulent strains.
Examples are:
a) Cholera:
- Most strains of cholera are harmless
- Responsible for producing harmful cholera
strain.

24. b) Scarlet Fever:
- Commonly affects children
- Signs and symptoms include sore throat, fever
and characteristic red rash
- Usually spread by inhalation
- There is no vaccine of this disease.
- Most of the clinical features are caused by
erythrogenic toxin, a substance produced by the
bacterium Streptococcus when it is infected by
Bacteriophage T12.

25. Classification of Virus