- Bacteriophage
Definition:
viruses which infect the bacteria are called bacteria
terio phages. 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 fibers 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. (
vibrio's, staphylococci, salmonella ).
4.
Bacteriophages
may provide the property of toxigenicity to some organisms such as C. diphtheria.
5.
Bacteriophages
have been extensively used in studying host-parasite relationships.
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