Bacteriophages, or phages, are viruses that infect bacteria. These microscopic entities play a fundamental role in the microbial world, influencing bacterial populations. Phages employ diverse strategies for replication within a host cell, including the lysogenic cycle.
What It Is
The lysogenic cycle is a bacteriophage life cycle where viral genetic material integrates into the host bacterium’s genome. Instead of immediately producing new viral particles, the viral DNA remains dormant. This integrated viral DNA is termed a “prophage.” A bacterium carrying a prophage is referred to as a “lysogen.” During this phase, the bacteriophage lies dormant within the host cell, allowing the bacterium to continue normal life and reproduction.
How the Cycle Progresses
The lysogenic cycle begins when a bacteriophage attaches to the surface of a bacterial cell, a process known as adsorption. Specific receptor sites on the bacterial cell wall facilitate this binding. The phage then injects its genetic material, DNA or RNA, into the host cell’s cytoplasm.
Once inside, the viral DNA integrates into the host bacterium’s chromosome. This integration is often facilitated by integrase, a phage-encoded enzyme. The integrated viral DNA, now a prophage, replicates passively with the host chromosome during cell division, ensuring its propagation without producing new phage particles.
Under environmental stress, such as UV radiation or specific chemicals, the prophage can excise itself from the host chromosome. This process, known as induction, activates the integrated viral DNA. Upon induction, the phage enters the lytic cycle, producing new phages and ultimately lysing the host cell.
Comparing Lysogenic and Lytic Cycles
The lysogenic cycle differs significantly from the lytic cycle, another common bacteriophage replication strategy. In the lytic cycle, viral DNA immediately takes control of the host cell’s machinery, rapidly replicating and producing new viral particles. This culminates in the destruction and lysis of the host cell, releasing new phages.
Phages capable of both cycles are “temperate phages,” like phage lambda which infects E. coli. Phages that only undergo the lytic cycle, causing immediate cell destruction, are “virulent phages.”
The primary difference lies in the host cell’s fate: in lysogeny, the host cell survives and reproduces, passively propagating the viral genome. Conversely, the lytic cycle results in the swift destruction of the host cell.
Why Lysogeny Matters
The lysogenic cycle holds broader biological significance, influencing bacterial evolution and pathogenicity. It facilitates genetic transfer, specifically transduction, where phages transfer bacterial genes between bacteria. This horizontal gene transfer contributes to bacterial diversity, adaptation, and the spread of antibiotic resistance.
Prophages can carry genes encoding toxins or virulence factors, transforming harmless bacteria into pathogenic ones. For example, Vibrio cholerae acquires its cholera toxin genes from a lysogenic bacteriophage. E. coli O157:H7 also becomes pathogenic due to toxins encoded by integrated prophages.
Understanding these cycles is relevant to medical applications like phage therapy, which explores using bacteriophages to treat bacterial infections. Manipulating induction conditions could help control bacterial populations.