Fundamentals of Microbiology

1.5.3 Lytic cycle of T – even bacteriophage

1. Attachment:

During this process, an attachment site on the virus attaches to a complementary receptor site on the bacterial cell. Attachment is established by the formation of weak bonds between the tail fibres of T – even phages and complementary receptor sites on the bacterial cell wall.

2. Penetration

After attachment, phage injects its DNA into the bacterium. An enzyme, phage lysozyme is released from phage’s tail, which breaks down a portion of the bacterial cell wall; the tail sheath contracts to force the tail core through the cell wall and phage DNA enters the bacterial cell. The capsid remains outside.

3. Biosynthesis

Once the phage DNA has entered cytoplasm of the host cell, host protein synthesis is arrested. Phage uses the host cell’s nucleotides and its enzymes to synthesis phage DNA. Phage DNA is transcribed into mRNA and synthesis of enzymes and capsid protein occurs. During this stage, only separate components (DNA, protein) are formed. This period during viral multiplication where complete virions are not yet present is called the eclipse period.

4. Maturation

In this process, DNA and capsids are assembled into complete virions

5. Release

During this final stage, lysozyme is synthesized within the cell which causes the bacterial cell wall to breakdown. This results in the release of bacteriophages.

The time from phage attachment to release is known as burst time and is usually from 20 to 40 min. The number of newly synthesized phage particles released from a single cell is called burst size and it ranges from 50 to 200.