In this article we will discuss about:- 1. Meaning of Bacteriophages 2. Ultrastructure of Bacteriophages 3. Life Cycle.
Meaning of Bacteriophages (Bacterial Viruses):
The viruses which attack, reproduce and lyse bacteria are called bacteriophages. The term bacteriophage literally means, “eater of bacteria”. The bacteriophages do not eat bacteria but infect and kill them. Twort discovered in 1915 and Herelle studied them more fully two years later in 1917.
The bacteriophage which infects the colon bacterium (Escherichia coli) is called a coliphage. Several strains of coliphage are known. The best known have been designated T2, T4, T6 or collectively T-even phages. They are perhaps the best known structurally.
Ultrastructure of Bacteriophages:
A diagrammatic sectional view of T-4 phage is shown in Fig. 19.4.
The bacteriophage particle resembles a tiny sperm. It has a head and a tail. In addition there is some sort of attachment region adapted to stick to the surface of the host cell. The virus particle of T-even phage is about 200 to 280 m µ in length.
The virus particle is thus too small to be seen with the best of light microscopes. Nevertheless, pictures of virus particles are obtained by the use of electron microscope.
The head of T-even phage is hexagonal in outline and bears numerous facets. It consists of protein coat surrounding a core of genetic material which in this case is DNA molecule. The coiled coil of a single thread-like double-stranded macro-molecule of DNA is packed tightly in the head.
It is about 50 microns long. The phage DNA is said to lack the base cytosine. Instead it has another base hydroxymethyleytosine (HMC). The protein coat is built of globular proteins.
The cylindrical tail entirely consists of a protein sheath surrounding an empty core. The tail sheath can contract longitudinally. The attachment apparatus of the phage consists of six long, slender protein fibres known as the tail or caudal fibres.
They arise from a plate at the basal end of the tail. The tail fibres normally remain twined inside the core of the tail. Extended they help to attach the phage particle to the coli cell.
Life Cycle of a Virulent Bacteriophage:
The coliphage particle is carried close to the coli cell. The tail plate and the caudal fibres anchor the phage virion to the host cell wall at a specific site which the tip of the phage tail happens to touch.
With the attachment of the phage to the specific receptor site on the host cell starts the life cycle of the phage. The next step is the injection of phage DNA. Within a few seconds an opening is formed in the host cell wall at the point of attachment by the enzyme in the tip of the tail.
The elastic protein sheath of the tail now contracts driving, the hollow core into the host cell, like a hypodermic syringe, through the opening (Fig. 19.6). The phage DNA is then forced down the hollow core and is injected into the host protoplast.
The protein coat of the phage remains sticking as an empty shell on the outside (B). The steps outlined above constitute the adsorption and penetration phase or infection phase in the life cycle of the bacteriophage.
Next follows the latent period phase or the period of eclipse. During the eclipse period which is usually about 12-22 minutes no virus DNA as such can be seen in the host cell. It disappears as an independent infective agent.
It seems that the phage DNA within the host cell enters the nuclear or chromatin body of the host cell and takes control of the cell’s genetic machinery in a manner unknown at present. The replication of bacterial DNA and synthesis of bacterial enzymes ceases immediately.
Under influence of phage DNA, the chromatin body of the host which also contains DNA is broken down and its contents are dispersed within the first few minutes after infection (B).
The bacterial enzyme systems are either commandeered or converted into systems capable of elaborating phage components from the cell contents and ingredients absorbed from the medium (C).
Phage DNA is replicated in the region of nuclear or chromatin body of the host cell. The host cell’s ribosomal factory in cytoplasm is caused to elaborate new phage coat protein.
The next step in the phage life cycle is the assembly of the phage components into new phage particles (D). As the elaborated particles of the phage DNA leave the chromatin region of the host cell, the new phage protein quickly aggregates round them to constitute new stable virions of T-even phage.
Assembly of phage units starts about 12-22 minutes after infection. The tail is finally added to the phage head. Within about half an hour after infection the host cell contains two to three hundred or even more particles of T-even phage.
The period of production of new phage particles is called vegetative phase. It comprises the replication of UNA, synthesis of phage protein and assembly of phage components into new phage particles.
The final step in the phage life cycle is the liberation phase (E). The host cell or whatever is left of it suddenly bursts to release the phage progeny. The process of disintegration or destruction of coli-phage cell is called lysis. The phage which brings about lysis or destruction of the host bacterium is called a virulent phage.