Short Answer
Bacteriophages – viruses that infect bacteria – can follow one of two major reproductive strategies. In the lytic cycle, the phage hijacks the host’s machinery, produces many new virus particles, and then bursts the cell open (lyses it). In the lysogenic cycle, the phage DNA integrates into the bacterial chromosome and replicates silently with the host until a trigger causes it to switch to the lytic mode.
Quick visual comparison
| Feature | Lytic cycle | Lysogenic cycle |
|---|---|---|
| Primary outcome | Cell death by lysis and release of progeny virions | Phage genome persists as a prophage; cell remains alive |
| Timing of replication | Immediate after infection | Delayed; replication occurs only after induction |
| Genetic integration | None (viral DNA stays episomal) | Integration into host chromosome (prophage) |
| Typical examples | Phage T4 infecting E. coli | Phage λ infecting E. coli |
| Induction triggers | Not applicable | UV radiation, chemicals, stress, or spontaneous excision |
| Ecological role | Rapid reduction of bacterial populations | Horizontal gene transfer, bacterial immunity (lysogenic conversion) |
Step‑by‑step of the lytic cycle
1. Attachment and DNA injection
The phage tail fibers recognize specific receptors on the bacterial surface and bind tightly. The tail contracts, creating a channel through which the phage genome is injected into the cytoplasm [1].
2. Host takeover
Early phage genes encode proteins that shut down host transcription and translation, redirecting ribosomes to synthesize viral proteins.
3. Genome replication and capsid assembly
Viral DNA is copied many times; capsid proteins self‑assemble into empty shells, which are then filled with newly made genomes.
4. Maturation and lysis
Endolysins and holins degrade the bacterial cell wall, causing the cell to burst and release thousands of progeny virions [1].
Step‑by‑step of the lysogenic cycle
1. Attachment and DNA injection
Identical to the lytic start – the phage binds and injects its DNA [1].
2. Integration (lysogenization)
Special integrase enzymes mediate recombination between phage DNA and a specific site in the host chromosome, forming a prophage.
3. Replication with the host
The prophage is copied each time the bacterial genome replicates, so the viral genome is passed to daughter cells without killing the host.
4. Induction to the lytic pathway
Stressors such as UV light or certain chemicals damage bacterial DNA, triggering the SOS response. This can cause the prophage to excise, resume viral gene expression, and enter the lytic cycle [3][4].
Why the distinction matters
Understanding the two cycles explains many natural and applied phenomena:
- Phage therapy: Virulent (lytic‑only) phages are preferred because they guarantee bacterial killing.
- Bacterial evolution: Prophages can carry toxin genes or antibiotic‑resistance determinants, a process called lysogenic conversion.
- Molecular genetics tools: The λ lysogenic system underlies many cloning vectors and inducible promoters used in the laboratory.
Self‑check
Choose the correct statement about a lysogenic phage:
- The infected cell is immediately lysed.
- The phage genome integrates into the host chromosome.
- No viral proteins are ever produced.
Answer: 2. Integration occurs; the cell remains viable until induction.
Next steps
Continue with a deeper look at “Temperate phages and lysogenic conversion” to see how prophages influence bacterial pathogenicity.
FAQ
What are the main differences between the lytic and lysogenic cycles?
In the lytic cycle, the phage rapidly replicates, produces many progeny virions, and lyses the host cell to release them. In the lysogenic cycle, the phage genome integrates into the bacterial chromosome as a prophage, replicates silently with the host, and only later may be induced to enter the lytic cycle.
How does a bacteriophage decide to enter the lytic versus lysogenic pathway?
The decision depends on multiple factors, including the physiological state of the host, multiplicity of infection, and environmental cues. Favorable growth conditions often lead to the lytic pathway, while stressful or nutrient‑limited conditions can favor lysogeny.
What triggers induction of a prophage into the lytic cycle?
Induction is typically triggered by DNA‑damaging stressors such as UV radiation, certain chemicals, or the bacterial SOS response, which activate prophage excision and entry into the lytic program.
Why are lytic phages preferred for phage therapy?
Lytic (virulent) phages guarantee bacterial killing because they do not integrate into the host genome and therefore do not risk transferring undesirable genes such as toxins or antibiotic‑resistance determinants.
How does lysogenic conversion affect bacterial pathogenicity?
When a prophage carries genes that encode toxins, antibiotic‑resistance factors, or other virulence determinants, the infected bacterium can acquire new pathogenic traits, a process known as lysogenic conversion.
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