“Bacteriophages are the viruses that infect bacteria.”

What is Bacteriophage?

A bacteriophage is a virus that infects a bacterial cell and reproduces inside it. They vary a lot in their shape and genetic material.

A bacteriophage may contain DNA or RNA. The genes range from four to several thousand. Their capsid can be isohedral, filamentous, or head-tail in shape.

Bacteriophage Structure

The bacteriophage consists of a polyhedral head, a short collar and a helical tail.

  • Head- The head consists of 2000 capsomeres with double-stranded DNA enclosed within.
  • Tail- The tail consists of an inner hollow tube which is surrounded by a contractile sheath with 24 annular rings. The distal end consists of a basal plate with tail fibres at each corner. The bacteriophage attaches to the bacteria with the help of these tail fibres.

Also Read: Overview of Microbes

Bacteriophage Life Cycle

Bacteriophage exhibits two major types of life cycles:

  • Lytic Cycle or Virulent Cycle
  • Lysogenic Cycle or Temperate Cycle

Lytic Cycle

In the Lytic Cycle, a bacteriophage infects a bacteria and kills it to release progeny virus. This cycle takes place in the following steps:


The bacteriophage attaches itself on the surface of bacteria. This process is known as adsorption. The tips of the tail fibres attach to specific receptors on the surface of the bacterial cell.


The tail sheath of the phage contracts after adsorption. The base plate and the tail fibres are attached firmly to the bacterial cell. The phage muramidase weakens a part of the cell wall and the hollow core is pushed downwards through it. The DNA is injected inside the bacterial cell.

Synthesis of Phage Components

The components of new virus particles are produced after the nucleic acid is released into the cell. The sub-units of phage head, tail and late protein then appear. The synthesis is carried out by specific enzymes called early proteins. The nucleus and the cytoplasm also contain the components of a phage.

Maturation and Assembly

On maturation, the head and tail protein of phage DNA assemble and each component of phage DNA is surrounded by a protein coat. Ultimately, the tail structures are added forming a virion.


The infected bacterial cell is lysed releasing the progeny phages. The phage enzymes weaken the cell wall of bacteria during replication.

Lysogenic Cycle

In this, the phage becomes integrated with the chromosome of the host cell and is known as a prophage. This prophage is transmitted to progenies at the time of cell division during reproduction in bacteria. The bacteria carrying a prophage without being lysed is called “lysogenic bacteria”.

When the lysogenic bacteria multiply, the prophage might be lost due to excision.

Also Read: Virology

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