Definition: Bacteriophage (Bacterio: means bacteria and Phage: means invaders or eaters) is a virus that attacks the bacterial cell. These phages show a parasitic relationship with the bacterial host cell. Bacteriophages not only replicate within the host cell (bacteria) but also controls the machinery of the host. These are ubiquitous in nature.
History of Bacteriophage
Bacteriophage was first studied by F.W. Twort (1915). Later the term ‘bacteriophage’ was given by Felix de Herelle (1917). After that, he found the bacteriophage useful for the patient suffering from dysentery caused by Bacillus dysenteriae and introduced ‘’Phage therapy’’.
Classification of Bacteriophage
On the basis of genome and morphology, the bacteriophage is classified by ICTV (International Committee on Taxonomy of Viruses).
On the basis of the genome: Bacteriophage can be classified into four types:
|Myoviridae||Genome: Linear, ds-DNA||Coliphage T2, T4, T6|
|Styloviridae||Genome: Linear, ds-DNA||Coliphage T1, T5|
|Pedoviridae||Genome: Linear, ds-DNA||Coliphage T3, T7|
|Corticoviridae||Genome: Circular, ds-DNA||Pseudomonas phage MP2|
|Tectiviridae||Genome: Linear, ds-DNA||Phage PRD1|
|Plasmaviridae||Genome: Linear, ds-DNA||Phage MV-L2|
|Microviridae||Genome: Circular, ss-DNA||Coliphage ɸX174|
|Inoviridae||Genome: Circular, ss-DNA||Coliphage fd|
|Cystoviridae||Genome: Segmented, ds-RNA||Phage ɸ6|
|Leviviridae||Genome: Linear, ss-RNA||Phage MS2|
Characteristics of Bacteriophage
- A bacteriophage is smaller than the bacteria.
- The genetic material of bacteriophage can be either DNA or RNA and linear or circular.
- Its genome size is about 49kb.
- The bacteriophage size ranges from 25-200nm in length.
- Bacteriophage can be either infectious or non-infectious to the host cell.
Structure of Bacteriophage
The structure of bacteriophage includes the following components:
- Hexagonal in shape
- Prismoid structure
- Surrounded by an envelope refers as “Capsid”
- Forms by identical protein subunits called ‘’Capsomeres’’.
- Composed of around 2000 capsomeres.
- It can be either DNA or RNA.
- The structure can be linear or circular
- Tightly packed inside the head
- 50nm long
- Also refers as “Collar”
- Connects head and tail
- Circular plate-like structure
- It is like a hollow tube
- Surrounded by a protein sheath
- Composed of around 144 protein subunit
- Highly contractile in nature
- Contains 24rings
- Hexagonal in shape
- Present at a distal end
- Attached to the base plate
- Long, thread-like filaments
- Found 6 in number
- Size: 130x2nm
- Also refers as ‘’Tail pin”
- Recognizes the receptor sites of the host cell
Life cycle of bacteriophage
It is of two kinds:- Lytic and Lysogenic cycle
It also refers as ‘’Virulent or Infectious cycle”. Phages involve in this refers as “lytic phage”. These phages infect or kill the host cell that is why it is known as a virulent phase.
It includes the following steps:-
- Cell lysis
Attachment: In this step tail pin or spikes recognizes the receptor sites on the host bacterial cell and attaches to it. After that, the tail fibres of the bacteriophage gets attached to the surface of the bacteria.
Adsorption: In this step, the amino groups of the host cell reacts with the carboxyl group of the bacterial cell and vice versa. After the reaction, the tail fibres secrete “Lytic enzyme”. This enzyme creates a hole in the host cell through which genetic material passes into the host cell cytoplasm.
Penetration: This step is also known as “Injection”, where the phage genetic material penetrates into the genetic material of the host.
Replication or biosynthesis: After the penetration, the mRNA moves out of the nucleus to the cytoplasm and the virus genetic material degrade the host genetic material. Then mRNA transcribes and translates to form protein capsid which leads to the biosynthesis of other components.
Assembly: It also refers as “Maturation”. At this stage, all the components of bacteriophage assemble to produce new daughter or progeny virions.
Lysis: At this stage, the virus kills the bacterial cell by cell lysis and releases about 100-200 progeny virions.
It also refers as “Temperate or Non-infectious cycle”. The phages that participate in this cycle are known as “Lysogenic phage”. These phages do not kill or infect the bacterial cell and that is the reason why it is called non-infectious phase.
It includes the following steps:-
- Incorporation of genetic material
- Cell division
Attachment: In this step, lysogenic phage first recognizes the receptors site of the host through their spikes. After recognition, tail fibres attach onto the host cell surface.
Penetration: It is also the same as the lytic phase where tail fibres release a lysogenic enzyme to create a pore, by which its genetic material goes into the cytoplasm of the host cell.
Incorporation of genetic material: It also refers as “Integration of genetic material”. At this stage, phage DNA incorporates into the genetic material of the ‘host genetic material’ which forms a complex that is “Prophage”. Prophage is the Temperate phage which is non-active i.e. not able to produce new progenies.
This step is simply the recombination of phage and the host’s genetic material, which does not involve the transcription and translation process.
Replication: During favourable conditions, prophage replicates when the bacterial genome replicates and pass onto the daughter cells.
Cell division: In this stage, a cell divides into two identical daughter cells.
Induction: It has two conditions:
- The prophage can remain in the dormant state inside the host cell
- Secondly, prophage can continue the lytic cycle by the induction of UV-rays or heat treatment.
Significance of bacteriophage
- It helps in the identification, classification and detection of pathogenic bacteria.
- It acts as a biocontrol agent, by killing bacteria that involves soil and water pollution.
- To study the concept of evolution it acts as modal organisms.
- Uses in genetic engineering.
- In space microbiology, it Used as “Radiation detector”.
- Uses in the treatment of many diseases caused by bacteria known as “Phage therapy.
- Plays a major role to control bacterial plankton growth.
- In horticulture, Uses as a spray to protect plants and vegetables.
- Uses as biocides such as disinfectant to clean up the environmental surfaces. E.g. In hospitals.
Therefore, the bacteriophage shows a wide range of ecological, molecular and biomedical significance.