Bacteriophages, commonly known as phages, are specialized viruses that infect and replicate exclusively within bacterial cells. Phages are composed of a nucleic acid genome encased in a protein shell called a capsid. Phages have two distinct strategies for replication: the lytic cycle, which results in the destruction of the host cell, and the lysogenic cycle, which allows the viral genome to lay dormant. Both cycles begin identically before their paths diverge to achieve different reproductive outcomes.
Shared Entry: Attachment and Genome Injection
The initial phases of infection are shared by both lytic and lysogenic pathways, focusing on gaining access to the bacterial cell’s interior. This process starts with attachment, where the phage physically binds to the exterior of the host cell. The phage uses specialized proteins on its tail fibers to recognize and connect with specific receptor molecules on the bacterial surface, such as proteins or sugar transporters. This interaction is highly specific, meaning a phage typically only infects one species or strain of bacteria.
Successful attachment triggers the second step, penetration, which involves injecting the viral genetic material into the host’s cytoplasm. For many phages, the tail sheath contracts, driving a hollow core tube through the bacterial cell wall and membrane. The phage’s DNA or RNA genome is then propelled through this tube into the host cell, while the empty protein capsid remains outside. Once the viral genome is free within the host cell, the decision between the two life cycles must be made.
The Lytic Path: Replication and Host Cell Destruction
If the phage genome commits to the lytic pathway, it immediately hijacks the cellular machinery for rapid reproduction. The viral genes synthesize early proteins, including enzymes that quickly degrade the host cell’s DNA, eliminating the bacterial genome’s control over its resources. The host’s ribosomes, amino acids, and energy reserves are then exclusively directed toward manufacturing viral components.
This phase, known as biosynthesis, involves the replication of hundreds of copies of the phage genome and the synthesis of structural proteins for the new capsids and tails. Maturation or assembly occurs spontaneously, where the newly created genetic material is packaged into the protein heads, and tails are attached to form complete, infectious phage particles. The final stage, lysis, involves the production of late-stage enzymes, such as lysozyme, which break down the bacterial cell wall. The resulting osmotic pressure causes the weakened cell to burst, releasing progeny phages into the environment to infect neighboring cells.
The Lysogenic Path: Integration and Dormancy
In contrast to the lytic cycle, the lysogenic path allows the phage genome to avoid immediately taking over the host cell. Instead, the viral DNA is integrated directly into the host bacterial chromosome, a process often facilitated by a phage-encoded enzyme called integrase. Once integrated, the viral genome is referred to as a prophage.
The prophage remains in a state of latency, where its genes for replication and lysis are actively suppressed, particularly by a protein known as the repressor. The host cell, now called a lysogen, continues to live and reproduce normally. Every time the bacterial cell divides, it replicates the prophage along with its own DNA, ensuring the viral genome is passed down to all daughter cells. This integrated state often provides the host cell with immunity against further infection by the same type of phage.
Environmental Triggers for Cycle Choice
The choice between the lytic and lysogenic cycles is influenced by the environment surrounding the infected cell. A key factor is the Multiplicity of Infection (MOI), the ratio of phages to bacterial cells. When the MOI is high (many phages infecting a single host), the phages often favor lysogeny to avoid rapid depletion of the host population.
Conversely, a low MOI or abundant nutrient availability, suggesting a healthy, rapidly dividing bacterial population, favors the lytic cycle for rapid reproduction. If a lysogen encounters environmental stressors, such as UV radiation, DNA-damaging chemicals, or nutrient starvation, the prophage can be triggered to exit the chromosome. This event, called induction, switches the dormant prophage back into the lytic cycle, leading to the destruction of the stressed cell and the release of new phages.