Viruses: Life Cycles of Bacteriophage.

Introduction:

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism, having the capacity to infect all types of life forms, from animals and plants to microorganisms.
It has the smallest size in the microorganisms 20-300nm.
They lack essential metabolic machinery to synthesize their own essential proteins hence has to depend on the hosts.
Viruses hence are called obligate intracellular parasites.

Life cycle of a bacteriophage serves as a model for life cycle study of viruses as similar processes are observed in eukaryotic cells also.
This was initially studied in T even bacteriophages of E. coli.
Bacteriophages exhibit two types of lifecycles,

Also called “Virulent Cycle”.
In this cycle newly formed viruses release from the host cell causing lysis (Breakdown) of the host cell.
Each cycle of phage production requires 20 to 60 mins. And produces around 200 new bacteriophages.
This cycle represents steps that occur in the life cycle of all the viruses.

This is the first step in the virus life cycle and is also called “Adsorption”.
The virus particle comes in contact with the host cell by random collision.
The virus particle gets adsorbed on the host cell by its tail.
The degree of adsorption depends upon the attraction between the viral molecule and the chemical molecules present on the host cell: this shows the reason for host specificity of viruses.
Even flagella, pilli also can serve as a site of attachment.
In laboratories the naked viral nucleic acid can be injected in the host cell to induce the infection: This process is called “Transfection”.

After successful attachment of viral particles with the host cell the viral nucleic acid is injected in the host cell: The process is called “Penetration”.
The viral tail contains lysozyme which creates a hole in the host cell wall.
Through this hole the nucleic acid enters the host cell.
Usually the nucleic acid alone is capable of inducing the reproduction process.
The empty viral particle i.e. head, tail that remains outside the host cell is called “Shell or Ghost”.
Sometimes many bacteriophages attach to the host cell and injects their nucleic acid inside the host cell, in such cases due to the multiple hole formation on the cell wall host cell lysis takes place: The phenomenon is called “Lysis from without”.

The bacteriophages contain DNA as nucleic acid material.
Once inside the host cell the viral DNA produces “early mRNAs”which are translated into early viral proteins by bacterial ribosomes.
These early viral proteins shut down the host macromolecules production, degrade the bacterial DNA and start producing viral DNA.
Once the viral DNA is complete it forms “late mRNAs” which initiate synthesis of capsid proteins and then head, tail and other parts are formed.

The viral DNA, head, tail and other parts are synthesized separately in the bacterial cell.
The viral DNA starts condensing in a polyhedron and then gets packaged inside the head part.
Finally the tail and other parts are attached to the head.
This process of assembly of the viral particle inside the host cell is called “Maturation of Virus”.

Once the cell gets filled with progeny viral particles the cell bursts releasing the viral particles.
The lysis of cell walls is assisted by lysozyme production in the cell.
The released progeny of bacteriophages infect other bacterial cells.

The sequence of events happening in between injection of phage nucleic acid material in the bacterial cell to release new virions is called “Viral multiplication or Replication Cycle”.
This can be plotted as a graph as one step multiplication cycle and is also called as “One Step Growth Curve”.

The time interval between entry of virion inside the bacterial cell and appearance of the first intracellular virion particle is called “Eclipse Phase”.
The Eclipse phase tells us the time taken by the virus to synthesize its components.
The time interval between entry of the virion inside the bacterial cell to release of first virion is called “Latent Period”.
The period during which the number of released virions rises after release of first virion is called “Rise Period”.
The yield of phage per bacterial cell is called “Burst Size.”

After successful entry inside the bacterial cell the viral nucleic acid gets integrated with bacterial chromosomes, this integrated chromosome is called “Prophage”.
The prophage once formed behaves as a part of the bacterial chromosome that replicates with it, it is called as “Lysogeny”.
The bacterium carrying a prophage is called “Lysogenic bacterium”.
The prophage adds new properties to the lysogenic bacterium it is called “Lysogenic conversion / Phage conversion”.
The lysogenic bacterium becomes resistant to the reinfection from same phage or the related phages it is called “Superinfection immunity”.
Now every time the bacterial cell replicates the prophage replicates and remains dormant inside the progeny cells.
However, due to action of some chemicals or UV rays the prophage gets activated and enters the “Lytic Phase” this phenomenon is called “Spontaneous Induction of Prophage”.