Lysogenic infection is a viral strategy where the virus integrates its genetic material into the host cell’s genome. The viral DNA then exists in a dormant state, allowing the host cell to continue normal functions without immediate harm. This integrated genetic material is replicated along with the host’s own DNA, ensuring its propagation through subsequent cell divisions.
The Journey of a Lysogenic Infection
The process of a lysogenic infection typically begins when a virus, often a bacteriophage, attaches to the surface of a bacterial host cell. After attachment, the bacteriophage injects its genetic material into the bacterial cell’s cytoplasm. This injected genetic material then undergoes a crucial step: integration.
The viral DNA, using specific enzymes like integrase, inserts itself into the host bacterium’s chromosome. Once integrated, the viral genome is referred to as a prophage. The host cell continues to live and reproduce normally, with the prophage being replicated along with the bacterial chromosome and passed down to all daughter cells. This state of coexistence can persist for many generations, without the virus causing any immediate disease or cell destruction.
The prophage can be activated to exit the host genome. Environmental stressors, such as exposure to UV radiation, certain chemicals, or even changes in temperature and nutrient availability, can trigger this activation, known as induction. Upon induction, the prophage excises itself from the bacterial chromosome and initiates a different viral life cycle that leads to the production of new viral particles.
Key Components of Lysogeny
Understanding lysogenic infection involves recognizing several specific components. A bacteriophage is a type of virus that infects bacteria. The host cell is the bacterial cell that harbors the viral genetic material.
A prophage is the bacteriophage genome after it has integrated into the host bacterial chromosome. This integrated genetic element remains dormant within the host’s DNA and is replicated along with it. The bacterial cell that carries this integrated prophage is known as a lysogen. Lysogens are characterized by their ability to carry the viral DNA without being immediately lysed, and they can pass this prophage to their offspring.
Lysogenic Versus Lytic Cycles
Viruses, particularly bacteriophages, can follow two primary life cycles: the lytic cycle and the lysogenic cycle. The lytic cycle is characterized by rapid viral replication within the host cell, leading to its destruction and lysis (bursting) to release newly formed viral particles. In this cycle, the viral DNA typically remains separate from the host’s genome and takes over the cell’s machinery.
In contrast, the lysogenic cycle involves the integration of viral genetic material into the host cell’s DNA, where it remains dormant. The host cell survives and continues to divide, replicating the integrated viral DNA along with its own. The key distinction is that the lytic cycle results in host cell death, while the lysogenic cycle allows the host cell to survive. While some phages are exclusively lytic, temperate phages possess the ability to undergo either pathway.
Why Lysogeny Matters
Lysogeny holds significant implications for bacterial evolution and broader biological systems. The integrated prophage can introduce new genetic information into the bacterial host, a process known as lysogenic conversion or phage conversion. This genetic transfer can confer new traits to the bacterium, such as enhanced virulence, making a previously non-pathogenic bacterium capable of causing disease. For instance, certain bacterial toxins, like the cholera toxin, are encoded by genes carried on prophages.
Lysogeny can also facilitate the transfer of genes that contribute to antibiotic resistance among bacteria. Prophages act as mobile genetic elements, contributing to horizontal gene transfer, which is a key mechanism for bacteria to acquire new genetic traits rapidly. This ability influences bacterial fitness, adaptation to various environments, and the emergence of new pathogenic strains. Understanding lysogeny is important for comprehending how bacteria evolve and adapt, and it also offers avenues for potential applications in areas like phage therapy.