This document summarizes key aspects of bacteriophages. It discusses the history of their discovery by Twort and d'Herelle in the early 1900s. Key terms are defined, including plaque, PFU, MOI, EOP, prophage, and lysogenic bacteria. The lytic cycle is explained in 7 steps, from attachment to lysis and release of progeny. Applications of phages include phage antibacterial therapy, use of phage components, phage-mediated vaccines, and phage typing for diagnostics.

1. PRESENTED BY,
B.KRISHNAKUMAR,
I-M.Sc.MICROBIOLOGY.
DR.N.G.P. ASC

2. History
Edward Twort (1915) and Felix d'Herelle (1917)
independently reported isolating filterable entities
capable of destroying bacterial cultures and of
producing small cleared areas on bacterial lawns.
It was F d'Herelle, a Canadian working at the
Pasteur Institute in Paris, who gave them the name
"bacteriophages"-- using the suffix phage (1922).

3. Glossary
 PFU:
Plaque forming unit
 MOI:
Multiplicity of infection, the ration of phage particles to bacteria
 EOP:
Efficiency of plating, the ration of the plaque titer to the number of
phage particles
 Prophage:
State of phage co-existing with host
 Lysogenic bacteria:
Term of bacteria carrying prophage
 Phage conversion:
Phenotype change in lysogenic bacteria

4. Plaque
 Plaques are clear zones formed in a lawn
of cells due to lysis by phage.
 At a low multiplicity of infection (MOI) a
cell is infected with a single phage and
lysed, releasing progeny phage which can
diffuse to neighboring cells and infect
them, lysing these cells then infecting the
neighboring cells and lysing them.
 It ultimately results in a circular area of
cell lysis in a turbid lawn of cells.

5. One step growth experiment,
 In this experiment, susceptible
bacterium (E. coli) is mixed with
bacteriophage (T2) and is allowed
for a short period to get attached
with the host cells.
 The culture is then diluted much,
thereby any virus particle if released
by lysis of host cell will not be able
to infect new cells.
 The number of phage particles
released from bacteria is
subsequently determined at
dif­ferent time intervals
by plaque count.

6. Lytic cycle of phage
1
2
3
4
5 6 7

7. Kinetics of phage infection
 0 min. Attachment of phage to a susceptible E. coli cell.
 1 min. Inject DNA into cell
 1-7 min. Transcribe and translate early genes
 block bacterial DNA synthesis and degrade host chromosomal DNA
 block transcription of host mRNAs
 block translation of host proteins
 small amounts of early proteins produced (catalytic functions)
 transcription from single phage genome
 7-15 min. Replication of phage DNA
 10-20 min. Translation of phage late proteins (structural)
 transcribed from new phage DNA (many copies of template)
 need large amounts of these proteins to build new virions
 18-25 min. Assembly of new phage particles (end of eclipse period)
 25 min. Lysis of host cell and release of progeny (end of latent
period)

8. Infection processes
1. Attachment of virion to
cell
2. Entry of viral nucleic acid into
host cell (with or without other
virion components)
3. Early viral proteins synthesized
(required for genome replication)
4. Genome replication
5. Late proteins synthesized
(capsid proteins)
6. Assembly of progeny virions
7. Release of infectious progeny virions

9. Assembly of phage

10. Lysogenic cycle

11. Applications of phages.
 PHAGE ANTIBACTERIAL THERAPY
 USE OF PHAGE COMPONENTS
 PHAGE-MEDIATED VACCINE
 PHAGE BASED DIAGNOSTICS LIKE PAHGE TYPING.
 PHAGE AS A VEHICLES TO VACCINE DELIVERY.
 ALSO SOME ENVIRONMENTAL ASPECTS.