Lytic cycle of bacteriophage (Virus): Introdcution, and Stages


Bacteriophages are viruses that only infect bacteria which is also known as phages.

They have a unique structural and life cycle and are highly abundant and diverse in nature such as water, soil, and the human body. It can infect and replicate within bacterial cells.

The term “bacteriophage” originates from the Greek words “bacterio,” meaning bacteria, and “phagein,” meaning to eat.

Bacteriophage, named T4 is a virus attack Escherichia coli. It completes its lifecycle inside the bacterial cells. Virus exhibit two types of cell cycles-lytic life cycle and lysogenic cycle.

Lytic Cycle:

It includes adsorption, penetration, formative phase, maturation and lysis. With lytic phages, bacterial cells are broken open (lysed) and destroyed after immediate replication of the virion. As son as the cell is destroyed, the phage progeny can find new hosts to infect. An example of a lytic bacteriophage is T4, which infects E. coli found in the human intestinal tract. Lytic phages are more suitable for phage therapy.

Some lytic phages undergo a phenomenon know as lysis inhibition, where completed phage progeny will not immediately lyse out of the cell if extracellular phage concentration is high.

Stages of the Lytic Cycle:


Bacteriophage attachment sites bind to host bacterial receptor sites.  Although some can attach to flagella or pili, most bacteriophages adhere to the bacterial cell wall. Only particular host bacterial strains can be bound by particular bacteriophage strains which is called viral specificity.


A bacteriophage enzyme “drills” a hole in the bacterial cell wall in the case of bacteriophages that adhere to the bacterial cell wall, and the bacteriophage then injects its DNA into the bacterial cytoplasm. Some bacteria-eating bacteriophages achieve this by forming a sheath that forces a hollow tube into the bacterium. The eclipse period starts at this point. These hollow organelles allow the entry of the bacteriophage genomes that attach to flagella or pili.

Stages of the Lytic Cycle

Fig: Stages of the Lytic Cycle


The DNA of the host cell is destructed and the metabolism of the cell is mediated to start the biosynthesis of the phage. The replication of the phage DNA takes place within the cell, producing novel phage proteins and DNA. The viral DNA directs the host cell to produce viral components, including new phage DNA, viral proteins, and viral enzymes.


Complete bacteriophage will be formed accordingly by the newly synthesized viral components. The viral DNA is packaged into the newly formed viral capsids, while the tail fibres and other structural proteins are assembled to form the tail apparatus. This stage often involves the cleavage of viral precursor proteins and the assembly of individual viral particles.

Lysis and Release

As the phage matures and assembles, lytic enzymes are released that result the host cell to lyse.

That activity of lytic enzymes consequence in the lysis of bacterial cell wall that release the newly generated virions. Consequently, the mature phage particles are freed, ready to infect fresh bacterial cells and restart the lytic cycle.

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