Replication cycle of Bacteriophages

 Reproduction (Replication cycle):

Bacteriophages exhibit two types of replication cycle – virulent or lytic cycle and temperate or lysogenic cycle (Fig. 10.6).

I. Virulent or lytic cycle:

The phages undergoing lytic cycle are called lytic phages or virulent phages, e.g., T-series bacteriophages. In lytic cycle, a lytic phage infects and kills the host cell to release progeny virions.

The whole process involves following steps:

(a) Adsorption or infection

(d) Virion assembly

(b) Penetration or injection

(e) Lysis or release

(c) Synthesis of phage components

Step-1. Adsorption or infection:

The lytic cycle begins with a collision between T-phage virion and a susceptible host cell i.e. Escherichia coli. The process of attachment of a virion on the host cell surface is called adsorption. The tips of tail fibers bind or adsorb to specific receptors on the surface of E. coli.

The viral receptors may be F-pili, lipoproteins, iron transport proteins etc. The T-phage virion adsorb to specific receptors by the tip of tail fibers. For example, T4 and T7 coliphages bind to lipopolysaccharides.

Step-2. Penetration or Injection:

The tail fibers of virion bend to bring the spikes and basal plate in contact with the surface of bacterial wall. The tail sheath contracts so that the hollow tail core (inner tube) penetrates the bacterial wall and injects the viral genome into the cytoplasm. After penetration, the empty capsid that remains outside the bacterium is called the ghost or doughnut.

Step-3. Synthesis of phage components:

Immediately after penetration, the phage DNA (genome) synthesizes early proteins. Some early proteins break down the bacterial (host) DNA and take the control of the bacterial cell machinery. The other early proteins used as enzymes for replication of phage DNA. The newly synthesized phage DNAs produces late proteins, which are the protein subunits of the phage capsid (head and tail).

Step-4. Virion assembly:

The capsid proteins assemble to form empty head and a condenced viral DNA is packed inside it. Finally the separately assembled tail joins to head to form a daughter or progeny virion.

Step-5. Lysis or release:

During assembly of progeny virions, the bacterial cell becomes spherical. The phage enzymes weaken the cell wall which ultimately burst or lyse to release about 100-200 progeny virions.

II. Temperate or lysogenic cycle:

The phages that exhibit lysogenic cycle are called temperate phages or non-virulent phages. For example, λ, (Lambda)- phages attacking, E. coli. During lysogenic cycle, the phage DNA integrates into the bacterial DNA and is now called as prophage. The host bacterium containing prophage is called a lysogenic bacterium or lysogen. The prophage passively replicates along with the host DNA for many generations. When a lysogenic bacterium exposed to UV-light or a chemical, the prophage withdraw from the host DNA to undergo lytic cycle. This conversion of a prophage into a lytic phage is called induction.