🔁 Lytic and Lysogenic Cycles

After attaching to a bacterial host and introducing its nucleic acid, a bacteriophage may follow different multiplication strategies. The two classic pathways are the lytic cycle and the lysogenic cycle. Understanding these cycles is essential for microbiology, bacterial genetics, and virus-host interactions.

Lytic Cycle

In the lytic cycle, the phage multiplies actively inside the host cell and eventually destroys the bacterium by lysis.

Lytic phages are therefore often called virulent phages.

Main Stages of the Lytic Cycle

1. Eclipse Phase

Soon after nucleic acid entry, no complete infectious phage particles can be detected. During this period:

• host machinery is taken over
• phage nucleic acid directs synthesis
• early proteins and nucleic acid replication begin

2. Intracellular Accumulation Phase

Structural components are synthesized and assembled into mature phage particles inside the bacterial cell.

3. Lysis and Release

The host cell is broken open by phage-directed lytic functions, and newly formed phages are released to infect other bacteria.

Plaque Assay

Lytic phages are commonly studied using the plaque assay.

A plaque is a clear zone formed when phages lyse bacteria growing on a lawn. Each plaque originates from one infectious phage particle, commonly expressed as a plaque-forming unit (pfu).

Lysogenic Cycle

In the lysogenic cycle, the phage does not immediately destroy the host. Instead, its nucleic acid becomes incorporated into or maintained with the bacterial genetic system in a dormant or repressed form.

Such phages are called temperate phages.

Important Terms in Lysogeny

Prophage

The phage genetic material present in the repressed state within the bacterial cell is called a prophage.

Lysogen

A bacterial cell carrying a prophage is called a lysogen.

In this state, the host cell may continue to grow and divide while carrying the phage genome.

Events Leading to Lysogeny

The general process includes:

• entry of phage nucleic acid
• stabilization or integration of the phage genome
• repression of most phage genes

The prophage is then replicated along with the host system.

Induction: Termination of Lysogeny

The lysogenic state can be terminated when the host experiences stress or adverse conditions.

Common inducing factors include:

• UV radiation
• ionizing radiation
• mutagenic chemicals
• physiological stress

Under such conditions, repression may be lost, the prophage becomes active, and the phage shifts into lytic multiplication.

Lytic vs Lysogenic Outcome

The outcome depends on regulatory control inside the infected cell.

• if phage genes for active multiplication dominate, the lytic cycle follows
• if repression and stable maintenance dominate, lysogeny follows

This choice is one of the most important ideas in phage biology.

Lysogenic Conversion

Lysogeny may alter bacterial characteristics because some phage genes can be expressed in the host.

This is called lysogenic conversion.

Its significance is great because phage-carried genes may change traits such as:

• antigenic properties
• toxin production
• pathogenicity

Thus, a bacterium can gain new properties because of prophage carriage.

Significance of Lytic and Lysogenic Cycles

These cycles are important because they help explain:

• viral multiplication
• bacterial population control
• transfer of traits
• gene regulation
• bacterial virulence changes

They are central not only to virology but also to microbial genetics.

Summary Cheat Sheet

• The lytic cycle ends in active multiplication and host-cell lysis.
• The eclipse phase is the early hidden period before mature phages are assembled.
• Plaque assay is used to detect and count lytic phages.
• In the lysogenic cycle, phage DNA persists in a repressed form as a prophage.
• A bacterium carrying a prophage is called a lysogen.
• Induction shifts a lysogenic phage back to the lytic cycle.
• Lysogenic conversion can alter bacterial properties such as virulence and toxin production.

References

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