In this article we will discuss about Bacteriophages:- 1. Meaning of Bacteriophages 2. Morphology of Bacteriophages 3. Chemistry 4. Mode of Infection.
Meaning of Bacteriophages:
Viruses parasitic on bacteria are called bacterial viruses or bacteriophages or just phages. The word bacteriophage is derived from the Greek and means ‘eater of bacteria’. They were discovered by the French scientist d’herelle in 1917.There are many varieties of bacteriophages.
Usually each kind of bacteriophage will attack only one species or even only one strain of bacteria. Their size and shape varies from species to species. Some phages are spherical, some comma-shaped whereas majority of them have tadpole-like appearance.
T-bacteriophages were first isolated in America from sewage. They infect the colon bacillus Escherichia coli which lives in mammalian intestine. They have been given the type numbers 1 to 7 (T1, T2 etc.). So they are divided into two main categories, T-even (T2, T4 and T6) and T-odd (T1, T3, T5, T7). Bacteriophages can be distinguished by their serological properties.
Morphology of Bacteriophages:
The morphology of the bacteriophages has been best studied by electron microscopy. It is tadpole-shaped with polyhedral head, a short neck and collar and a straight tail. The head is bipyramidal hexagonal in shape and measures 950 x 650 Å. The contents of head are enclosed by a membrane (capsid) about 35 Å thick.
The capsid is made up of about 2000 capsomeres (proteins) and each protein subunit has molecular weight of about 80,000. The head encloses a linear double stranded DNA, which contains more than 75 genes. The DNA remains greatly folded.
The tail is in the form of a hollow cylinder. It consists of a central hollow core surrounded by a spring-like contractile sheath. The sheath is formed of 144 subunits which are arranged in a hollow cylinder consisting of 24 rays of six subunits each. Through the central space of the core, the phage chromosome travels into the host cell.
The core with its contratile sheath rests on a hexagonal end plate. This plate bears six short spikes and six long tail fibres measuring about 1300 x 20 Å with a kink on the middle. Each tail fibre and spike is a bundle of polypeptide chain with a molecular weight about 100,000.
In the lamda coliphage the head capsid is a regular icosahedron. In the coliphages T3 and T7 the tail is short and stubby. The coliphages T1 and T5 have no tail sheath and their fibres are rudimentary.
Chemistry of Bacteriophages:
Physical and chemical studies have established that the T – even phages are composed wholly of DNA and protein. DNA constitute 40% by weight and protein remaining 60%. DNA occurs within the head. Head membrane, sheath and tail fibres are composed of different proteins. In bacteriophage MS2, the genetic material is RNA instead of DNA.
Hershey and Chase (1952) labelled the protein by radioactive sulphur (35S) and DNA by radioactive phosphorus (32P) in T2 phage and established that the genetic material of the virus is its DNA and not the protein. This agrees with Avery, MacLeod and McCarty’s discovery with Diplococcus pneumoniae, a bacterium.
Mode of Infection of Bacteriophage:
There are two types of bacterial infections:
Virulent infection and temperate infection.
1. Virulent Infection:
T-even phages (T2, T4 and T6) of Escherichia coli are examples. They multiply as soon as they enter the host cell, resulting in lysis or breakdown of host cell. Such viruses are called lytic viruses. The mode of infection is called virulent.
There are four steps in the life cycle of lytic bacterial viruses. They are Adsorption, Injection, Replication and Lysis.
(a) Adsorption:
At the time of infection, a bacteriophage becomes anchored to the bacterial wall by its tail fibres and tail spikes so that the tail and head are perpendicular to the surface.
(b) Injection:
Within a few minutes after attachment, the contractile sheath of the tail shortens, to less than half its original length. An enzyme of the core of the tail digests part of the bacterial cell wall. It results in the production of a hole and finally, the phage DNA is injected into the bacterial cell. The capsid of phage remains outside the bacterial cell.
(c) Replication:
The bacterial DNA is disturbed; manufacture of bacterial protein stops and the viral genes takes over. Among the first proteins to be made are the enzymes needed for viral DNA replication. This happens within the first 5 minutes after break through. Three minutes later another set of genes directs the synthesis of structural proteins that will form the head components and tail components.
The first completed virus particles appear 13 minutes after infection. This process continues for another 12 minutes until about 200 such particles have been completed and the raw materials of the bacterial cell have been virtually exhausted.
(d) Lysis:
A viral enzyme, a lysozyme, then appears and attacks the cell wall, liberating the new viral particles.
2. Temperate Infection:
The best studied temperate phage is the lamda phage of the bacterium E. coli. In this virus do not cause lysis of the host cell in the usual way. The viral chromosome becomes integrated with the host chromosome.
Detailed study on the life cycle of lamda phage indicate that on entering the host cell, the phage DNA has two options. It may immediately multiply and start lytic cycle (virulent infection) described earlier. But other times (lysogenic cycle), the viral chromosome becomes a part of the host chromosome and is called the prophage. When the viral DNA is incorporated into the host DNA it behaves like a gene on the genetic map of the host. It replicates along the host DNA.
Thus lamda phage exists in two forms, the virion and the provirus. The virion is the normal condition with a DNA core and a protein coat. The provirus is the intracellular form in which the viral DNA is a gene on the DNA of the host. Bacteria containing prophage are called lysogenic bacteria. Viruses whose chromosomes becomes prophage are called lysogenic viruses.
The lamda phage DNA is inserted into the E. coli DNA at a specific site between the gal operon and the bio operon. The insertion of a phage DNA requires a specific enzyme which has been called integrase. Removal of phage DNA from the bacterial chromosome requires integrase and a second viral gene enzyme called excisionase.