Bacteriophages (phages) are viruses that infect and replicate within bacteria. They are the most abundant biological entities on Earth, playing a major role in regulating bacterial populations. Upon infecting a host cell, a phage chooses one of two distinct reproductive strategies: the lytic cycle or the lysogenic cycle. This choice determines the host bacterium’s fate, leading to either rapid destruction or temporary coexistence.
The Lytic Cycle: Immediate Replication and Host Destruction
The lytic cycle is the fast, destructive reproductive pathway, which is the sole option for obligately lytic phages (virulent phages). The process begins when the phage attaches to specific receptor sites on the bacterial cell surface. Following attachment, the phage injects its genetic material—either DNA or RNA—into the host cell, while its protein coat remains outside.
Once inside, the viral genome immediately hijacks the host cell’s machinery, directing it to synthesize new viral components. Early viral genes express enzymes that degrade the host’s DNA, shutting down bacterial operations and redirecting resources toward viral replication. The cell rapidly manufactures hundreds of copies of the viral genome and the necessary protein subunits.
The components then spontaneously self-assemble into mature phage particles inside the cytoplasm. The final stage is lysis, where the phage produces an enzyme, such as lysozyme, which breaks down the bacterial cell wall. This causes the cell to burst open, releasing the newly formed phage progeny to infect surrounding bacteria. The entire lytic cycle is a rapid process resulting in the immediate death of the host cell.
The Lysogenic Cycle: Integration and Dormancy
The lysogenic cycle is an alternative strategy available only to temperate phages, which can choose between the two life cycles. This pathway is characterized by latency, where the phage genetic material does not immediately commandeer the host cell. After attachment and injection, the phage DNA integrates directly into the host bacterium’s chromosome.
Once integrated, the dormant viral DNA is referred to as a prophage, and the host bacterium carrying it is called a lysogen. A repressor protein encoded by the prophage maintains this state of dormancy by preventing the expression of the lytic genes that would otherwise lead to cell destruction. The bacterium continues to live, grow, and divide normally, but now, every time the host cell replicates its own DNA, it passively replicates the integrated prophage as well.
Passive replication allows the viral genome to be distributed vertically to all daughter cells, ensuring the phage’s long-term persistence in the bacterial population. The prophage can switch back to the lytic cycle in a process called induction. Induction is often triggered by environmental stress signals, such as UV radiation or DNA-damaging agents, which activate the host cell’s SOS response system. This stress causes the repressor protein to be inactivated, allowing the prophage to excise itself from the chromosome and initiate the destructive lytic pathway.
Comparing Viral Strategies
The difference between the lytic and lysogenic cycles lies in the ultimate fate of the host cell and the state of the viral DNA. The lytic cycle involves immediate and complete takeover of the host machinery, leading to the rapid death of the bacterium and the release of numerous viral particles. This strategy maximizes viral output quickly but eliminates the host population.
In the lysogenic cycle, the host cell survives and continues to reproduce, acting as a stable, long-term reservoir for the viral genome. The phage DNA exists as a prophage, physically integrated into the host chromosome, replicating passively alongside it. This integration contrasts sharply with the lytic cycle, where the viral DNA remains separate and free-floating in the cytoplasm, actively directing the cell’s functions.
Viral propagation involves active, mass production in the lytic cycle versus passive replication in the lysogenic cycle. Lytic phages (virulent phages) are committed to the immediate destructive pathway. Temperate phages possess the genetic switch to choose the lysogenic pathway, allowing them to persist undetected until induction switches them back to the lytic cycle. The lysogenic strategy provides latency, allowing the virus to survive when host cells are scarce, while the lytic strategy aims for rapid population growth.