Bacteriophage, Types, Composition and structure of Bacteriophage in Detail


Bacteriophage is a virus that infect the bacteria. This was discovered by Twort in 1915 in England and by Felix at the pasture institute in 1917 in Paris. The term bacteriophage was given by Herelle which mean bacteria eater.

Composition and structure of Bacteriophage:

Bacteriophage is also composed of nucleic acid cover with protein coat and capsid made up of sub unit of capsomere.

Ultrastructure of T-even Phage:

T- coliophage is tadpole shaped. It also consists of tail and head.


Head is form of a bipyramidal and hexagonal prism. Head consist of double standard viral DNA and cover with proteinaceous membrane.


  1. Tail is cylindrical in shape and connect with head, tail made up of four components.
  2. Central tube is hollow, and infection viral DNA pass through in this tube.
  3. The central part is made of proteinaceous sheath.
  4. The sheath is connected to thin disc (collar) at the head side.
  5. The distal end of sheath connects to hexagonal plate that make a complex structure.
  6. The plate has a pin at every side.
  7. Pin is connected to long six tail fiber which are attached to the wall of host.

Bacteriophage Types:

Virulent/ Lytic phages:

Avirulent/Temperate phages:

Virulent/ Lytic phages:

These viruses destroy their bacteria cell after infection. they produce lager number of viruses in host cell. The host cell burst of reproduction and release new generation in host cell and infect other bacteria cell.

Avirulent/Temperate phages:

These viruses not destroy the host cell and not harm it. They reproduce itself to many generations in host cell with the help of nucleic acid without damage. After many generations temperate stage convert in to lytic phages.

Bacteriophage Life Cycle:

In bacteriophage two different types of life cycle were observed. These are given below.

Types of life cycle:

  1. Lytic Life cycle (Destroy the host cell)
  2. Lysogenic Life cycle (Not destroy host cell)

Lytic Life cycle:

Lytic bacteriophage life cycle consists of following steps.

  1. Adsorption
  2. Penetration
  3. Viral DNA in Host
  4. Bacteriophage assembly
  5. Release of bacteriophage

Adsorption phase:

  1. First step of lytic life cycle is adsorption in which phase infection causing virus to enter in bacteria cell by adsorption.
  2. Ionic bound develop between host and infection causing agent at receptor site which interact with specific protein in the capsid.

Penetration Phase:

  1. Penetration is the second step of lytic life cycle.
  2. In this step tip of the virus tail is attach to bacteria at receptor site.
  3. The tail of virus bend and base plate of virus attach to the cell surface.
  4. The tail injects hollow spike into the cell. Lysozyme helps for this process.
  5. Virus nucleic acid inject inside the cell and protein coat remain outside.
Enter into Host

Viral DNA in Host:

  1. In this phase viral DNA and take control of metabolism and bacteria start viral enzyme using ribosome of host and production in host cell.
  2. Nucleases break down the host cell DNA.
  3. Viral mRNA is formed and start synthesis viral protein that remake head, fiber and tail in host cell for further process.
  4. Now viral DNA replicate itself and increase amount.
  5. This phase is called transcription.

Bacteriophage Assembly:

  1. Nucleic acid and protein synthesis in previous phase.
  2. In this phase bacteriophage assemble.
  3. More than 200 new bacteriophages are assembling before starting initial infection in first 25 mint.

  Release of Bacteriophage Phase:

  1. After completing assemble the bacteria cell start bursting and releasing new phages in bacteria cell and infect new cell of bacteria.
  2. After this new cell begin again.
  3. The time taken for lysis this period is called latent period.
Lytic Life Cycle Diagram in Bacteriophage

Lysogenic Life Cycle of Bacteriophage:

  1. In this cycle temperate phages did not take charge over the function of gene cell.
  2. They incorporated into the DNA of host cell and become prophage in the host chromosome and act as gene.
  3. In this life cycle bacteria reproduce and mobilize normally, and viral DNA transmitted to each daughter cell through all generation.
  4. The viral DNA is removing from the host chromosome and start lytic life cycle.
  5. This process is called spontaneous induction.
  6. Lysogeny best studies in coliphage lambda.
  7. The multiplication of phage is observed inside the infected cell.
  8. Genes are responsible for virus multiplication and lysis.
  9. The virus possesses the gene that code for repressor protein that make cell resist to lysis starting or lytic cycle.
  10. The repressor protein isolate and purified. It is acid in nature.
  11. Ultra violet light helps to expose to phages they show the synthesis of protein in host.
  12. This protein has proteolytic activity and bind the lambda prophage and induce the lysis.   

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