Bacteriophage Reproduction: Lytic Cycle.

Bacteriophages are the viruses that infect the bacteria (host). There are two different types of bacteriophage based on the mode of reproduction: Temperate bacteriophages and Virulent bacteriophages.

Fig 1: Structure of T4 particle (M. Rossmann lab, Purdue Univ).

The temperate phages replicate through lysogenic cycle. They infect and inject their nucleus acids into the bacteria, which gets integrated into the bacterial genome. The integrated phage nuclei acid is replicated along with each round of bacterial replication. The viral infection does not kill the bacterial cell (until lytic cycle is induced). These bacteriophages are avirulent or not as severe as lytic phages.

Fig 2: Electron micrographs of bacteriophage T4.(A) Extended tail fibers recognize the bacterial envelope. (B) The internal tail tube is visible protruding from the contracted tail sheath.

The virulent phages replicate through Lytic Cycle. This mode of reproduction is virulent and causes lysis of bacterial cells. This occurs in five different stages –

1• Attachment/ Adsorption:

This is the first step in the viral infection, wherein the virus comes in contact with its compatible host. It consists of 2 stages: reversible and irreversible attachment. Initially the phage comes in reversible (weaker) contact and interact with surface particles of bacterial cells. These surface molecules include lipopolysaccharides, oligopolysaccharides, pili and flagellar proteins. The host cell molecules are recognised by viral adhesins, mostly present on phage tails. The host and viral interaction is specific and responsible for host specificity. The adsorption becomes irreversible when the tail pins attaches to the host surface.

2• Penetration:

The adsorption leads to relay of activities in the virion (the extracellular infectious form of virus). To start with, the adsorption results into production of hole in viral capsid envelope. This is followed by the tubular tail (fig 2B) penetrating the bacterial cell via outer membrane and peptidoglycan layer. Usually the penetration hole is produced by the viral encoded lysozyme carried on the phage tail. The tubular tail later fuses with the internal membrane of the bacteria and releases nucleic acid into cytoplasm (syringe like fashion). RNA viruses may need to inject RNA-dependent-RNA-polymerase as well. The penetration is an active process requiring energy in form of ATP. The protein coat remains outside attached to the host cell wall and is called “Ghost”.

Fig 4: Host cell entry by prokaryotic viruses. Yellow lines- membrane bilayers. Green lines- peptidoglycan layers. Red- Membrane-active peptides and proteins. TMP- tape measure protein. LPS- lipopolysaccharide.

(Just for info: Image source of Fig 4 and more on penetration here)

3• Synthesis of Phage Nucleic acid and structural components

After penetration the virus takes over the control of host cell. The host functions like DNA replication, transcription, translation and cell division are inhibited within 1–3 min after infection. This is done with help of early genes product of the virus. These immediate-early proteins make the conditions favourable for phage production. These include nucleases that break down host DNA and enzymes that alter host RNA polymerases.

Delayed early genes produce polymerases, ligases and phage DNA constituents like replace cytosine with 5- hydroxymethylcytosine, enabling protection from host restriction enzymes.

The viral structural components are synthesized by the late genes. This include head, tails, fibers and viral lysozymes, which accumulate within the host cell.

4• Maturation

The phage nucleic acid and structural proteins are assembled to form a complete or mature phage particle. This process is called maturation.

The structural protein initiate assembly around the nucleic acid molecule. Initially the head is assembled then the tail is attached. Eventually, the tail fibres are attached forming complete viral particle.

5• Lysis and release of new virions:

After about 25 mins of infection, around 200 new phage particles are assembled. The phage-encoded membrane protein forms lesion in the host cell membrane. Then the phage-encoded endolysin disrupts the cell wall, leading to lysis. On lysis the amount of progeny phage particles released is known as the burst size.