Bacteria, fundamental to nearly every ecosystem, engage in complex interactions with other microorganisms. These interactions include a fascinating dynamic with viruses that specifically target them, known as bacteriophages. Such relationships highlight the remarkable adaptations governing microbial life.
Understanding What a Lysogen Is
A lysogen is a bacterial cell that harbors the genetic material of a bacteriophage, a type of virus that infects bacteria. This viral genetic material, typically DNA, does not immediately take over the cell’s machinery to produce new viruses. Instead, it integrates into the bacterium’s own genome or exists as a stable, extrachromosomal element. In this state, the bacterium continues to live, grow, and divide, often appearing unaffected by the viral DNA.
This distinguishes a lysogen from a bacterium undergoing a typical viral infection where the virus rapidly replicates and destroys the host. The integrated phage DNA replicates along with the bacterial chromosome, passing from one bacterial generation to the next. A lysogen represents a stable, often long-term, partnership between a bacterium and a temperate bacteriophage.
How a Lysogen Forms
The formation of a lysogen begins when a specific type of bacteriophage, known as a temperate phage, infects a bacterial cell. Unlike virulent phages that immediately initiate a destructive lytic cycle, temperate phages possess a crucial choice upon infection. They can either enter the lytic pathway, leading to rapid viral replication and host cell destruction, or the lysogenic pathway, which results in a more subdued interaction.
In the lysogenic pathway, the temperate phage injects its genetic material into the host bacterium. This viral DNA then integrates into the bacterium’s chromosome, a process often mediated by specific enzymes called integrases. The integrated viral DNA is now referred to as a prophage. Once integrated, the prophage’s genes responsible for viral replication and cell lysis are largely repressed, ensuring the bacterium’s survival. The bacterial cell can then continue to divide, faithfully replicating the prophage along with its own genetic information.
The Prophage Within the Lysogen
While residing within the lysogen, the integrated viral DNA, or prophage, is largely dormant and does not actively produce new viral particles. Despite its quiescent state, the prophage is not entirely inert; it can confer new characteristics to its bacterial host, a phenomenon known as lysogenic conversion. These new traits can offer advantages to the bacterium, sometimes influencing its interaction with its environment or other organisms.
One effect is superinfection immunity, where the presence of the prophage prevents the same or a closely related type of bacteriophage from infecting the lysogen. This immunity is often mediated by repressor proteins produced by the prophage, which block the expression of incoming phage genes. Prophages can also carry genes that encode toxins or other virulence factors, transforming a harmless bacterium into a pathogen. For instance, the bacteria responsible for diphtheria, botulism, and certain severe forms of E. coli infections (producing Shiga toxins) acquire their ability to cause disease from specific prophages.
From Lysogen to Lysis: The Induction Process
A lysogen’s stable state is not necessarily permanent; the integrated prophage can be “induced” to re-enter the lytic cycle, leading to the destruction of the bacterial host. This process, known as induction, is typically triggered by specific environmental stressors that signal unfavorable conditions for the host bacterium. Triggers often include DNA-damaging agents like ultraviolet (UV) radiation, certain chemicals such as mitomycin C, or some antibiotics.
Upon induction, the prophage excises itself from the bacterial chromosome, becoming an independent genetic element. This excision is a precise process, often involving the same integrase enzymes that facilitated its initial integration. Once excised, the prophage reactivates its genes, initiating the rapid replication of its genetic material and the synthesis of viral proteins. This leads to the assembly of new bacteriophage particles within the bacterial cell, culminating in the lysis, or bursting, of the host bacterium and the release of numerous new phage progeny, ready to infect other bacterial cells.