Bacteriophages

Bacteriophages are bacteria infecting viruses. They are also called ‘phage’ or simply bacterial viruses as any group of viruses that infect bacteria are referred to as Bacteriophage. A bacteriophage is a virus that parasitizes bacteria and reproduces inside it. They are of different shapes and show genetic variations. They may contain DNA or RNA as genetic material and may have gene counts ranging from four to several thousand. The name bacteriophage describes an entity’s bactericidal ability and it translates to ‘’bacteria eater’’ in English. Not only do bacteriophages infect the bacteria but also archaea- the single-celled prokaryotic organisms.

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Characteristics of a Bacteriophage

• Several varieties of bacteriophages exist in the environment but one type can infect only one type or a few types of bacteria.

• They are classified into a number of Virus families. Examples include Inoviridae, Microviridae, Rudiviridae, and Tectiviridae.

• Like all other viruses, they are simple organisms consisting of a core of genetic material surrounded by a protein capsid.

• The genetic material can either be DNA or RNA in the bacteriophages.

• After infecting a cell, it completely takes control of the host cells and stops it from producing bacterial components, and forces it to produce viral components.

• They eventually bring about the lysis of the host bacterial cell.

• They are also involved in a process called transduction, in which Bacteriophages occasionally remove a portion of their host cell’s DNA and transfer this DNA into the genome of new host cells.

Structure of Bacteriophages

A typical bacteriophage is composed of a polyhedral head, a shirt collar, and a helical tail.

• The head of the phage consists of 2000 capsomeres with the genetic material- double-stranded DNA or Single-stranded RNA enclosed within the head.

• The tail is composed of an inner hollow tube that is surrounded by a contractile sheath with 24 annular rings. The distal end of the trail consists of a basal plate that has tail fibres at each corner.

Life Cycle of a Bacteriophage

After the phage infects the host cell and inserts its genetic material into the host cell, it follows either of the two life cycles, are-

1. Lytic Cycle (Virulent Cycle)

2. Lysogenic Cycle (Temperate Cycle)

Lytic Cycle

If they uptake the lytic cycle, bacteriophages infect the host cell and kill it to release progeny viruses. The steps involved in this cycle are as follows:

• Adsorption

This is the first step of infection by phage in which the bacteriophage attaches itself to the surface of the host cell or bacteria. For attachment to take place, the tips of the tail fibres attach to specific receptor sites on the surface of the bacterial cell.

• Penetration

In the next step, the tail sheath of the phage contracts after adsorption has taken place. The base plate and the tail fibres attach firmly to the bacterial cell surface. The phage lysozymes weaken a part of the host cell wall and the hollow core is pushed downwards through it. The phage DNA is then injected inside the bacterial cell.

• Synthesis of Phage Components

The components of new virus particles are produced after the genetic material of the phage is released into the host cell. The sub-units of phage then appear which includes the head, tail, and late protein. Early proteins and specific enzymes carry out the synthesis. Components of phage are also present in the nucleus and the cytoplasm.

• Maturation and Assembly

When the cells mature, the head and tail protein of phage DNA which is surrounded by a protein coat assemble. Ultimately, virion forms by the addition of tail structures.

• Release

The lysis of the bacterial cell takes place releasing the progeny phages. During the replication, phage enzymes weaken the cell wall of bacteria. 

Lysogenic Cycle

It is another pathway of viral reproduction in a host cell. In this phase the integration of phage nucleic acid into the host cell genome or the formation of a circular replicon in the cytoplasm of the host cell takes place. The host bacterial cell continues to live and reproduce normally in this phase. The genetic material of the phage also called prophage is transmitted to daughter cells at each subsequent cell division. The lysogenic cycle is different from the lytic cycle in the respect that the lysogenic cycle does not lyse the host cell straight away. The prophage may be converted into the lytic phase either naturally or artificially by physical or chemical agents. The bacteria carrying prophage viruses without being lysed are known as “lysogenic bacteria”.

In the event of the multiplication of lysogenic bacteria, the prophage might be lost due to excision.