Whilst the ultimate outcome of the lytic cycle is production of new phage progeny and death of the host bacterial cell, this is a multistep process involving precise coordination of gene transcription and physical processes.
The phage must identify a susceptible and suitable host bacterial cell to which it is able to attach. Following introduction of phage genetic material into the cell, the host genome is destroyed, and the phage utilizes the host cellular machinery to make copies of its own genome and synthesize the structural components.
New phage are assembled before the host cell finally lyses and the phage progeny released into the surrounding environment to find new target cells.
Steps of Lytic Cycle
- Adsorption
- Penetration
- Replication
- Maturation
- Release
#1. Adsorption.
Attachment sites on the bacteriophage adsorb to receptor sites on the host bacterium. Most bacteriophages adsorb to the bacterial cell wall, although some are able to adsorb to flagella or pili.
Specific strains of bacteriophages can only adsorb to specific strain of host bacteria. This is known as viral specificity.
#2. Penetration.
In the case of bacteriophages that adsorb to the bacterial cell wall, a bacteriophage enzyme “drills” a hole in the bacterial wall and the bacteriophage injects its genome into the bacterial cytoplasm.
Some bacteriophages accomplish this by contracting a sheath which drives a hollow tube into the bacterium.
This begins the eclipse period. The genomes of bacteriophages which adsorb to flagella or pili enter through these hollow organelles. In either case, only the phage genome enters the bacterium so there is no uncoating stage.
#3. Replication.
Enzymes coded by the bacteriophage genome shut down the bacterium’s macromolecular (protein, RNA, DNA) synthesis. The bacteriophage replicates its genome and uses the bacterium’s metabolic machinery to synthesize bacteriophage enzymes and bacteriophage structural components.
#4. Maturation.
The newly synthesized phage genome is then encapsulated within the phage particles. This packaging ensures that the genetic material is protected during the next steps of the viral life cycle.
Maturation involves the final steps of assembling functional phage particles, which then become ready for release. These particles contain the genetic information required for initiating infection in new host cells.
#5. Release.
The host cell, filled with replicated phage particles, finally undergoes lysis, causing death. The mature phage particles are thus released inside the host. A single bacterium on lysis releases 50 to 200 phages.
This release is often a violent process where the bacterial cell membrane is compromised, and the new phage particles are released into the surrounding environment. The released phages are now free to infect new bacterial cells, continuing the infectious cycle.