2. Out line of the presentation
īą Introduction
ī¯ Composition and Structure of Bacteriophages
ī¯ Infection of Host Cells
ī¯ Phage Multiplication Cycle
īŽ A. Lytic or Virulent Phages
īŽ B. Lysogenic or Temperate Phage
ī¯ Phage Typing
ī¯ Application of Bacteriophages
ī¯ Reference
3. Introduction
ī¯ Bacteriophage
īŽ Virus that infects bacteria.
īŽ Obligate intracellular parasites that multiply
inside bacteria by making use of some or all of
the host biosynthetic machinery.
ī¯ Twort and d'Herelle in 1915 and 1917
īŽ broth cultures of certain intestinal bacteria could
be dissolved by addition of a bacteria-free filtrate
obtained from sewage.
4. Diversity
ī¯ There are at least 12 distinct groups of
bacteriophages
ī¯ which are very diverse structurally and
genetically;
ī¯ the best known ones are the common phages
of E.coli:T4(lytic phage )&Lambda
Phage(Lysogenic phage)
ī¯ Phage Diversity.docx
5. Composition and Structure of Bacteriophage
A. Composition
ī¯ They all contain nucleic acid and protein.
īŽ Depending upon the phage, the nucleic acid can
be either DNA or RNA but not both.
īŽ Complexity vary from 3-5 average size gene
products code for over 100 gene
ī¯ The nucleic acids of phages often contain
unusual or modified bases.
īŽ These modified bases protect phage nucleic acid
from nucleases that break down host nucleic acids
during phage infection.
6. B. Structure
Bacteriophages come in many different sizes and
shapes.
ī¯ Size - T4 is among the largest phages; it is
approximately 200 nm long and 80-100 nm wide.
Other phages are smaller. Most phages range in size
from 24-200 nm in length.
ī¯ Head or Capsid -All phages contain a head
structure which can vary in size and shape.
īŽ Head acts as the protective covering for the
nucleic acid.
7. ī¯ Tail-Many but not all phages have tails
attached to the phage head.
ī¯ Is a hollow tube through which the nucleic
acid passes during infection.
ī¯ size of the tail can vary
ī¯ E.g. Complex phages like T4
īŽ The tail is surrounded by a contractile sheath
which contracts during infection of the bacterium
īŽ At the end the tail have a base plate and one or
more tail fibers involved in the binding of the
phage to the bacterial cell.
9. Infection of Host Cells
A. Adsorption
ī¯ Mediated by the tail fibers or by some analogous
structure on those phages that lack tail fibers and it is
reversible.
ī¯ The tail fibers attach to specific receptors on the
bacterial cell and the host specificity of the phage
ī¯ The nature of the bacterial receptor varies for
different bacteria(proteins on the outer surface of the
bacterium, LPS, Pili, and lipoprotein)
10. B. Irreversible attachment
ī¯ The attachment of the phage to the bacterium via
the tail fibers is a weak one and is reversible.
ī¯ Irreversible binding of phage to a bacterium is
mediated by one or more of the components of the
base plate.
ī¯ Phages lacking base plates have other ways of
becoming tightly bound to the bacterial cell.
11. C. Sheath Contraction
ī¯ The irreversible binding of the phage to the
bacterium results in the contraction of the
sheath (for those phages which have a sheath)
and the hollow tail fiber is pushed through the
bacterial envelope.
D. Nucleic Acid Injection
ī¯ The DNA/RNA ejected to the bacterial cell
through the hollow tube
īŽ The remainder of the phage remains on the
outside of the bacterium
12. Phage Multiplication Cycle
ī¯ Based on the multiplication cycle /event occur
during replication:-
A. Lytic or Virulent Phages
B. Lysogenic or Temperate Phage
13. A. Lytic or Virulent Phages
ī¯ Lytic or virulent phages are phages which can
only multiply on bacteria and kill the cell by
lysis at the end of the life cycle.
ī¯ Life cycle
īŽ 1. Eclipse period
īŽ 2. Intracellular Accumulation Phase
īŽ 3. Lysis and Release Phase
15. B. Lysogenic cycle/Temperate
ī¯ Either multiply via the lytic cycle or enter a
quiescent state in the cell
īŽ In this quiescent state most of the phage genes are
not transcribed
ī¯ Phage DNA in the repressed state is called a
prophage because it is not a phage but it has
the potential to produce phage.
ī¯ The cell harboring a prophage is termed a
lysogen.
16. Events Leading to Lysogeny:-The Prototype
Phage: Lambda
1. Circularization of the phage chromosome
2. Site-specific recombination
3. Repression of the phage genome
īą A phage coded protein, called a repressor, is
made which binds to a particular site on the phage
DNA, called the operator, and shuts off
transcription of most phage genes except the
repressor gene.
17. What determines whether the virus takes
Lytic or Lysogenic cycle?
ī¯ Determined by the concentration of the
repressor and another phage protein
called cro in the cell.
ī¯ If a promoter of repressor gene called C I
dominates the phage will progress to
lysogenic.
ī¯ If the an operator gene called cro dominates
the phage will progress to lytic cycle.
18. Events Leading to Termination
of Lysogeny
ī¯ Anytime a lysogenic bacterium is exposed to
adverse conditions, the lysogenic state can be
terminated. This process is called induction
ī¯ Adverse conditions lead to the production of
proteases (rec A protein) which destroy the
repressor protein.
īŽ This in turn leads to the expression of the phage
genes, reversal of the integration process and lytic
multiplication
20. ī¯ Used for identification and differentiating of
bacterial pathogen.
ī¯ Specific bacteriophage isolated is mixed with
known bacterial pathogen
ī¯ The lysis due to viral infection of the
bacterium is specific and important in
identification of the host.
Phage Typing
21. Plaque assay/Assay for Lytic Phage
ī¯ Lytic phages are enumerated by a plaque
assay
ī¯ A plaque is a clear area which results from the
lysis of bacteria.
ī¯ Each plaque arises from a single infectious
phage.
ī¯ The infectious particle that gives rise to a
plaque is called a pfu (plaque forming unit).
22. Application of Bacteriophages
ī¯ Model for animal virus transformation
īŽ Lysogeny is a model system for virus
transformation of animal cells
ī¯ Bacteriophage-based Diagnostics and Phage
Typing
ī¯ Phage as vehicles for vaccine delivery
ī¯ Phage display
īŽ A molecular technique used in synthesizing
polypeptides with novel characteristics.
23. Contâd
ī¯ Phage Therapy
īŽ Can any phage be used for developing
therapeutic phage preparation?
ī¯ Environmental Applications
īŽ Bio control for dangerous bacteria
24. Reference
1. Microbiology byte [Internet].[Last updated 2010, cited 2013]
http://www.microbiologybytes.com/virology/Phages.html
2. University of South Carolina School of Medicine: Microbiology and Immunology on
line [Internet].[Last updated 2010, cited 2013] Available from
http://pathmicro.med.sc.edu/mayer/phage.htm
3. Kenneth Todar.The Microbial WorldLectures in Microbiology by Kenneth Todar
PhD University of Wisconsin-Madison Department of Bacteriology [Internet].
[Last updated 2012, cited 2013] Available from
http://textbookofbacteriology.net/themicrobialworld/Phage.html
4. Madigan, Martinko, Stahl, Clark. Brock Biology of Microorganisms 13thEdition:
Virus and Virology: Pearson Education .Inc; 2008.
5. Elisabeth Kutter, Alexander Sulakvelidze. Bacteriophage Biology and Application:
CRC Press; 2005.
6. Irshad Ul Haq, Waqas Nasir Chaudhry, Maha Nadeem Akhtar, Saadia Andleeb and
Ishtiaq Qadri. Bacteriophages and their implications on future biotechnology: a
review. Virology Journal 2012; 9(9);1-8
7. Filippov AA, Sergueev KV, Nikolich MP (2013) Bacteriophages against Biothreat
Bacteria: Diagnostic, Environmental and Therapeutic Applications. J Bioterr Biodef
2013, S3 : 1-8.
Editor's Notes
1. Eclipse period During the eclipse phase, no infectious phage particles can be found either inside or outside the bacterial cell. The phage nucleic acid takes over the host biosynthetic machinery and phage specified m-RNA's and proteins are made. There is an orderly expression of phage directed macromolecular synthesis, just as one sees in animal virus infections. Early m-RNA's code for early proteins which are needed for phage DNA synthesis and for shutting off host DNA, RNA and protein biosynthesis. In some cases the early proteins actually degrade the host chromosome. After phage DNA is made late m-RNA's and late proteins are made. The late proteins are the structural proteins that comprise the phage as well as the proteins needed for lysis of the bacterial cell.2. Intracellular Accumulation Phase In this phase the nucleic acid and structural proteins that have been made are assembled and infectious phage particles accumulate within the cell.3. Lysis and Release Phase After a while the bacteria begin to lyse due to the accumulation of the phage lysis protein and intracellular phage are released into the medium. The number of particles released per infected bacteria may be as high as 1000.
The cell harboring a prophage is not adversely affected by the presence of the prophage and the lysogenic state may persist indefinitely.