- Bacteriophage
Definition: viruses
which infect the bacteria are called bacteriophages. these are usually
called as phages. Usually the bacteriophages are employed extensively in basic
molecular research and epidemiological tracings.
Morphology of
phages: it has been the unique form comprising of a genome
and surrounded by a protein coat known as phage capsid. The genome contains
single, linear, double stranded DNA molecule. Structurally, large phages
consist of head and tail.
The Head:
It is a hexagonal structure, comprising with tightly packed core of nucleic
acid and protein coat/capsid measures with the size 28-100 nm.
The Tail: it
is a cylindrical structure, which helps for the attachment of phage to the
host, thus it seems to be a channel for allowing the phage DNA into the host
cell. it consists of a hallow core and is surrounded by a contractile sheath and terminal base
plate in this plate pins or tail fibres or both are attached.
Chemical
structure: Chemically bacteriophages is protein in
nature and it contains only one type of nucleic acid which in majority of
phages is DNA, specifically a group of
phages which attack male strains of Esch. Coli containing only RNA. The n
nucleic acid makes about 50% of the dry weight of phages.
Life Cycle of
phages: Phages exhibit two different types of life cycles
namely;
a) Lytic
cycle
b) Lysogenic
cycle
a. Lytic
cycle: In this, the intracellular multiplication of the
phage occurs thereby the host bacterium is lysed and releases the virions. This
is also known as virulent cycle.
b. Lysogenic
cycle: In which phage DNA becomes integrated into the
bacterial genome and replicates with the bacteria without causing any harm to
the host cell. This is also called as temperate cycle.
Lytic cycle - The
lytic cycle of bacteriophages can be described in following steps:
Adsorption:
adsorption
is the initial step mainly depends upon the susceptibility of the bacterium to
the specific phages and the presence of chemical receptors on the surface of
the bacterium. In this step the phage gets attached on the host bacterium by
the help of tail. it is a very rapid
process and is completed within minutes.
Penetration:
penetration
is the next level mechanism. One the phage gets attached, the phage inject the
phage DNA into the periplasmic membrane
space of the bacteria. In most phages, the empty head and tail remains
outside the bacterium even after the injection of phage DNA. After entry, phage
DNA gets converted into circular form and the process is called as circulation
of phage DNA.
Eclipse
Phase: The
state in which the bacteria get infected with phage is not detectable for
several minutes. During this period a number of new proteins are being
synthesized which include enzymes necessary for synthesis of phage DNA. It is
also known as early proteins.
Synthesis
of phage components : After the eclipse phase, no further
synthesis of bacterial proteins takes place, instead , synthesis of protein
subunits of phage’s head and tail starts. These proteins are called as late
proteins.
Assembly
: In this step, the protein subunits of the phage head
and tail aggregate spontaneously (self-assembly ) to form the compact capsid.
Maturation:
it is the assembly of the phage components into the mature infective phage
particle.
Lysis
and liberation of new phages: Phage synthesis would
be continued until the cell disintegrates and liberating the virulent phages.
However the cell bursts due to the osmotic pressure after which the cell wall
has been weakened by the lysozyme. The period between the infection of bacterial
cell and the first release of infectious phage particles is known as latent
period. The average yield of progeny phages from the infected bacterial cells
is known as burst size .The period during which infectious phage released rise
is known as the rise period.
Lysogenic
Cycle: In the phase, the host cell is not affected even it
is prone to be infected with phage. Some phages on entering into the cell, do
not multiply in the manner as in virulent phages. Instead, these integrated
into the host chromo-some. This integrated genome of the phage is called as
prophage and the phenomenon is known as lysogeny. Bacteria which contain
prophage in their chromosome are
designated as lysogens and such phages are called as lysogenic or temperate
phages. certain new properties on the lysogenic bacterium. This is known as
lysogenic conversion or phage conversion. Some example are:
1. Phage
mediated conversion of somatic antigens of Salmonella
2. Phage
mediated toxigenicity of Coryne-
bacterium diphtheria
3. Phage
mediated toxicity in clostridium
botulinum
A lysogenic bacterium
is resistant to reinfection by the same or related phages. This is known as
superinfection immunity.
Bacteriophage
Typing: The bacteria strains can be identified
by bacteriophage typing method. In this typing method phage suspensions are
inoculated onto agar plates on which a lawn culture has been made with the
suspected pathogen. Susceptible bacteria are lysed by the phages, leaving clear
areas known as plaques. Based on this reaction, a bacterial species can be
divided into various types. This methods has been used in epidemiological
tracing of infections or outbreaks due to Staphylococcus aureus, Salmonella and
Vibrio cholera, etc.
Importance
of Bacteriophages :
1.
Bacteriophage can be used as carriers for
transmitting the gens from one bacterium to another- a process known as
transduction. This can be studied in drug resistance Staphylococcus aureus.
2.
Phage typing is used as an
epidemiological marker and it helps in establishing the path of transmission of
infectious agent and identifying the reservoir of infection.
3.
In order to subtyping of a species or
genus bacteriophage is used. ( vibrios, staphylococci, salmonella ).
4.
Bacteriophages may provide the property
of toxigenicity to some organisms such as C. diphtheriae.
5.
Bacteriophages have been extensively
used in studying host-parasite relationships.
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