They are one of the types of virus that attacks the bacteria and infects it.
It was first reported by Frederick Twort, a british biologist and later by Felix d’Herelle, a French microbiologist.
Diagram of phage
Characteristic features of a bacteriophage
- They have a typical structure of outer protein capsid enclosing a genetic material
- The genetic material can be DNA or RNA and might be ss or ds.
- The size of the nucleotides can vary from 5000 to 500, 000 nucleotides
- The genetic material can be circular or linear
- They are much smaller than the bacteria typically of size 20nm to 200nm.
- The prime source of these virions is the highly populated areas of bacteria, mostly the sea water where upto 9*108 virion per ml is found.
They are classified by the International Committee on Taxonomy of Viruses (ICTV) according to the morphology and characteristics.
There are two stages of bacteriophage replication
- lytic cycle
- lysogenic cycle
In this case the phage particle infects the bacterial cell and inserts the viral genetic material into it. The viral genetic material if is RNA uses reverse transcriptase enzyme to convert it to DNA and thus translates other building materials for a potential virion. The number of this virions increase at an exponential rate to burst the bacterial cell and set free to attack other baterial cells in the vicinity. This is a pretty fast rate of replication.
In this case the viral genetic material is incorporated into the bacteria’s own chromosome and thus is transferred to its progeny. When integrated with the host DNA the formation is called a prophage. And the phages able to cause lysogeny are called temperate phages. The virus remains dormant until the host conition deteriorates and once the stage reaches the viral DNA becomes active and thus causes lysis of the cell.
The prophages sometimes add more functions to the host cell. This is called lysogenic conversion. One famous example is the conversion of harmless viobrio cholerae by a phage into a highly virulent one. This is one reason why temperate phages are not used for phage therapy.
Diagram explaining the lytic and lysogenic cycle of a bacteriophage
More information at
One more classification of bacteriophage is a m13 bacteriophage
- It is a filamentous bacteriophage composed of circular single stranded DNA
- The length is 6407 nucleotides
- It is encapsulated in 2700 copies of the major coat protein P8, capped with 5 copies of two different minor coat proteins (P9, P6, and P3).
- The minor P3 attaches to the receptor at the tip of the host E.Coli.
- It is not lethal but causes plaques in the bacterial cell
- It is a non-lytic virus
- The M13 phage is used for many recombinant DNA processes due to its extreme size and the virus has also been studied for its uses in nanostructures and nanotechnology
The M13 infection cycle, the replication process
- In this cycle the DNA is put into the bacteria through the F-pilus.
- Once inside the cell the single stranded molecule acts as the template for the synthesis of a complementary strand, resulting in normal double stranded DNA.
- This molecule is not inserted in to bacterial genome but instead replicates until over 1000 copies are present in the cell.
- When the bacterium divides, each daughter receives copies of the phage genome. This continues to replicate thereby maintaining its overall numbers per cell.
- The new phage particles are continuously assembled and released.
For further information about M13 bacteriophage click HERE
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