Bacteriophage (Greek = Backterion = small rod, phagein = to eat) Viruses which are bacteria eaters are called bacterial viruses or Bacteriophages. They are obligate parasites of bacteria. Usually they are very minute and cannot be seen with ordinary light microscope. They are different from plant viruses. In environment, they are found in all those places in which bacteria are present, such as soil, dirty water, in intestine of animals and human beings, milk, vegetables, fruits, faeces, fertilizers, in rivers, in seawater etc.
Historical Background
The credit of bacteriophage discovery goes to Twort – d’Herelle (1917) who explained its structure in detai. He described that in a suspension of Staphylococcus albus when bacteriophages are added, all the bacteria disappeared and a transparent area appeared in the medium. The factor, which made the bacteria disappear, is called baeteriophage by Twort-d’Herelle. The phenomenon of disappearance of bacteria was also called Twort – de Herelle phenomenon (Twort-de-Herelle, (1915). After this, these workers not only isolated bacterial viruses, but also isolated them from blue-green algae, fungi and other microorganisms, and to express them a word phage is used. These viruses depend upon bacteria, blue-green algae, fungi and other microorganisms.
On the basis of the nature of the host they are named as:
ADVERTISEMENTS:
(a) Actinophugc
(b) Zymophage
(c) Cyanophage etc.
on the basis of the nature and type of the nucleic acid bacteriophages are classified into four types :
ADVERTISEMENTS:
1. Single Stranded DNA Phages: e.g., f x 174
2. Double Stranded DNA Phages: e.g., -T2, T4, T6, phages and T1 , T3 T5 and T7 phages or E. coli.
3. Single Stranded RNA Phages: e.g., DNA phage or E. coli.
4. Double Stranded RNA Phage:
ADVERTISEMENTS:
Structure of Bacteriophages:
Electron microscope depicts the correct and clear structure of bacteriophage.
The shape is like Tadpole or Sperm. It has two parts—anterior part head and posterior part tail. Head is hexagonal and is like a prism. Head is 90 to 95m in length and 60-65m in breadth. Tail is 100 m in length and 20-25m in breadth. It has 20,000 nucleotides and 600 —genes. Its DNA is double-stranded and its atomic weight is 120 x 106 Dalton units. This is covered with a covering called Capsid. Capsid is made up of protein and surrounds Nucleic acid core. With the help of osmotic shock phage particles without Nucleic acid can be obtained. The middle part of tail is called core, which is made up of DNA. It is surrounded by a protein covering. Shape of the tail is like a tube and is attached to the base of the head by a thin disc or collar. The diameter of tube is approximately 10 nm. From the end plate of the tail, several terminal fibrils are attached.
These fibrils help phage particles to attach themselves on the bacterial cell wall. In this way fibrils htdp in sticking like cement. End plate functions as tail pin and helps in forming a pore in the cell wall; this activity is supplemented by an enzyme. Phage DNA is injected into the bacterial cell through this pore.
ADVERTISEMENTS:
Researches in connection with bacteriophage have been carried out in detail on a strain of E. coli. This bacterial strain has been found to be sensitive to 7 strains of bacteriophages and diey have been named T1 to T7. T3 and T7 types of bacteriophages have rounded head and have comparatively smaller tail.
Some phage particles are the shape of a tadpole. Their heads are hexagonal and tail is comparatively longer. They are composed of protein and DNA. In addition, a special protein, one simple polypeptide, one or two free ammo acids and two purines are also present.
Growth Cycle and Multiplication in Bacteriophage :
Credit of detailed study of Bacteriophage multiplication or reproduction goes to American scientists, Max Delbruch and S. Luria and their coworkers and Andre Lwoff of France who have worked on T-phages of E. coli. They said that bacteriophage occurs in 3 forms such as extracellular virion, vegetative phage, and prophage. Vegetative phage and prophage both are intracellular and present in the form of DNA. All bacteriophages remain in the form of vegetative phage for some time in host cell but only ionic of them converts into prophage. The vegetative phages which form prophage are known as temperate phage and which do not from prophage are known as virulent phages.
ADVERTISEMENTS:
Bacteriophages are not active outside the host cell. Hence, they are known as resting particles. They are total parasites; that is why they can only multiply in living bacterial cells. They are only found in those places where their host is present, in presence of host bacterial cell they become active and invade the cell.