Bacteriophages are viruses that infect bacteria. They were discovered independently by Frederick W. Twort in England in 1915 and by Felix d’ Herelle at the Pasteur’s Institute in Paris in 1917. The term bacteriophage age was coined by D’ Herelle which means bacteria eater. Bacterial viruses are widely distributed in nature. Phages exist in almost all bacteria.
Structure and Composition
Like all viruses, the bacteriophages are composed of a nucleic acid core surrounded by a protein coat, the capsid made up of sub-units, the capsomeres.
Ultrastructure of T-even Phage
A T-even coliophage is tadpole shaped. It consists of a head and a tail.
The head is in the form of a bipyramidal hexagonal prism. The head consists of proteinaceous membrane enclosing a core of double-stranded viral DNA.
The tail is cylindrical attached to one end of the head. The tail is made up of four components.
It is a central helical hollow tube or core through which viral DNA passes during infection. The core is surrounded by a helical proteinaceous sheath. The sheath is connected to a thin disc or collar at the head end. The distal end of the sheath is connected to a hexagonal basal plate, which is of complex structure. The plate has a pin at every corner. The plate along with its pins is connected to six long thin tail fibers which are the organs of attachment to the wall of host cell.
Types of Bacteriophages
There are two main types of bacterial viruses:
Lytic or Virulent Phages
These are phages that destroy their host bacterial cell after infection. They inside the cell and produce a large number of viruses. The host cell bursts to new phages which infect other bacterial cells. They exhibit lytic cycle.
Temperate or A-virulent Phages
These are phages that do not harm and destroy the host bacterial cell. The viral acid is carried and replicated in the host bacterial cells without any damage for ty generations. However, temperate phages may spontaneously become virulent at some subsequent generation and lyse the host cells, i. e, exhibit lysogenic cycle.
Life Cycle of Bacteriophage
Two efferent types of life cycles are exhibited by bacteriophages: lytic cycle or lysogenic cycle.
Lytic Life Cycle
In lytic life cycle phage multiplies inside the host cell and the multiplication results in the lysis or disintegration of the host bacterium cell. The progeny is released to attack new bacterium cells.
The following steps can be noticed during the life cycle of a lytic bacteriophage:
The first step in infection of a host bacterial cell by a phage is adsorption. The virion attaches itself to host cell through ionic bonds or at more or less specific receptor sites which interact with specific proteins in the capsids or the virion.
The second step is called penetration. The tip of the virus tail is attached to the receptor sites on bacterial cell surface. The tail fibers bend to anchor the tail pins and base plate to the cell surface. The tail sheath contracts, forcing the hollow spike into the cell. Lysozyme in the base plate helps the process. The viral nucleic acid is injected into the host Cell. The protein coat remains outside the cell.
Establishment of Viral DNA in Host
The viral DNA takes control of the cell metabolism and directs the bacterium for production of viral enzymes using ribosomes of host. These include nucleases that break down host DNA. Viral mRNA is formed which directs synthesis of viral proteins that assemble heads, tails and fibers. The viral DNA replicates and its amount increases. This phase of life cycle is called transcription.
Assembly of Bacteriophage
While the synthesis of both structural proteins and nucleic acid is taking place, new phages begin to assemble. About 25 minutes after initial infection, some 200 new bacteriophages have been assembled.
Release of Bacteriophage
After assembly of phages, the bacterial cell bursts, release new phages to infect other bacteria and begin the cycle over again. The last phase is called assembly and release. The time taken from infection until lysis is called latent period.
Lysogeny or Lysogenic Life Cycle of a Bacteriophage
Not all infections of bacterial cells by phages terminate in lysis. In some cases, an entirely different relationship, known as lysogeny, meaning having potential for lysis, may develop between the virus and its bacterial host.
In lysogeny the viral DNA of the temperate phage, instead of taking over the function of the cell’s genes, is incorporated into the host DNA and becomes a prophage in the bacterial chromosome, acting as a gene. In lysogeny, the bacterium metabolizes and reproduces normally, the viral DNA is transmitted to each daughter cell through all successive generations. Sometimes, however, for reasons unknown, the viral DNA is removed from the host chromosome and the lytic cycle occurs. This process is called spontaneous induction. A change from lysogeny to lysis can sometimes be induced by irradiation with ultraviolet light or by exposure to some chemicals.
Lysogeny has been studied best in coliphage lambda. It has been observed that the multiplication of phage is repressed inside the infected cell, because the genes responsible for phage multiplication and lysis are switched off. The phage possesses a gene that codes for repressor protein which makes the cell resistant to lysis initiated either by the prophage or another lytic virus. The repressor protein has been isolated and purified. It is an acidic protein. When the phage lambda is exposed to ultraviolet light, this introduces the synthesis of a protein in the host cell. This protein has proteolytic activity cleaves the repressor protein preventing it to bind to the lambda prophage. This possibly induces lysis.