Bacteriophages

Bacteriophages are viruses that infect bacteria. They are commonly called as ‘phages’. Phages occur widely in nature in close association with bacteria. They can be readily isolated from feces, sewage and other natural sources of mixed bacterial growth.

By the process of transduction, phages play an important role in the transfer of genetic information among bacteria. The presence of phage genome integrated with bacterial chromosomes confers on bacteria certain properties by a process known as phage conversion.

Phages has been used as cloning vectors in genetic manipulations.

The presence of high concentration of phage particles up to 10^8 per ml in some natural wastes suggest that they may have a role in the control of bacterial population in the environments.

Phage property of having specificity of the host range, phage can help in the identification of bacteria by phage typing method.

Morphology

Certain bacteriophages that infect E. coli, called the T even phages (T2, T4, T6), have been studied by most scientist in great details. So, they serve as the prototype in describing the properties of bacteriophages.

T even phage have a complex and characteristic morphology.  They are called in tadpole shaped, with a hexagonal head and a cylindrical tail. The head consist of tightly packed nucleic acid (ds DNA) surrounded by a protein coat or capsid. The size of head may range from 28 nm to 100 nm. Opposite to it tail part is composed of a hollow core, a contractile sheath surrounding the core and a terminal base plate which has attached to it prongs, tail fibers or both.

there are some phages those are spherical or filamentous and possess ss DNA or RNA have also been studied and identified.

Life Cycle

Phages exhibit two different types of lifecycle. In the virulent or lyric cycle, intracellular multiplication of the phage culminates in the lysis of the host bacterium and the release of progeny virions. In the temperate or lysogenic cycle the phage DNA becomes integrated with the  bacterial genome, replicating synchronously with it, causing no harm to the host cell.

Lytic Cycle

Replication of a virulent phage can be considered in the following stages adsorption, penetration, synthesis of phase components, assembly, maturation and release of progeny phage particles.

A phage attaches to the surface of a susceptible bacterium by its tail. Adsorption is a specific process and depends on the presence of complementary chemical groups on the receptor sites of the bacterial surface and on the terminal base plate of the phage.

Under optimum conditions, adsorption is very rapid process. Certain cofactors such as cations, are necessary for the adsorption.

Host specificity of phage partial is determined at adsorption of phage.

Adsorption if followed by the penetration of the phage nucleic acid into the bacterial cell. The process of penetration resembles injection through a syringe.

The contractile tail sheath acts like a muscle and derived its energy from a small amount of adenosine triphosphate present on the tail of the phage. The phage DNA is injected on the bacterial body through the hollow core.

Penetration may be facilitated by the presence on the phage tail of lysozyme which produces a hole on the bacterial wall for the entry of the phage core. 

The complex structure of the phage particle is required only for the injection of the nucleic acid into the bacterial / host cell. The phage DNA alone is necessary for the initiation of the synthesis of daughter phages. After penetration, the empty head and tail of the phage remain outside the bacterium as the shell or ‘ghost’.

Phage DNA, head protein and tail protein are synthesized separately in the bacterial cell. The DNA is condensed into a compact polyhedron and ‘packaged’ into the head and, finally, the tail structures are added. This assembly of the phage components in to the mature infective phage particle is known as maturation.

Release of the mature progeny of phages typically occurs by the lysis of the bacterial cell. During the replication of phage, the bacterial cell wall is weakened and it assumes a spherical shape. Phage enzymes act on the weakened cell wall causing it to burst or lose resulting in the release of mature daughter phages.

The interval between the entry of phage nucleic acid into the bacterial cell and appearance of the first infectious intracellular phage particle is known as eclipse phase.

The interval between the infection of a bacterial cell and the first release of infectious phage particles is known as the latent period.

Lysogenic Cycle

Unlike virulent phages which do lysis of host cell, temperate phages enter into a symbiotic relationship with their host cells without destroying them. After entry into the host cell, the temperate phage nucleic acid become integrated with the bacterial chromosome.  The integrated phage nucleic acid is known as the prophage.

This phenomenon is called as lysogeny and a bacterium that carries a prophage within its genome is called alysogenic bacterium.