Lysogenic Phage: How It Alters Its Bacterial Host

Bacteriophages, often called phages, are viruses that specifically infect bacteria. They represent the most abundant biological entities on Earth. Among them, a group known as temperate or lysogenic phages employs a reproductive strategy that does not immediately result in the destruction of their bacterial host. Instead of rapidly multiplying and bursting the cell, these phages can enter a dormant state by integrating their genetic material into the host bacterium’s genome. This process allows the viral DNA to be copied and passed down through bacterial generations, awaiting specific signals to awaken.

The Lysogenic Life Cycle Explained

The journey of a lysogenic phage begins with attachment to a specific bacterial host, using its tail fibers to recognize and bind to receptor molecules on the cell surface. Following this docking, the phage injects its genetic material, typically DNA, into the bacterium’s cytoplasm, leaving its protein coat outside. Once inside, the lysogenic pathway diverges from the lytic cycle. Instead of seizing control of the cell’s machinery for immediate replication, the phage DNA navigates toward the host’s chromosome.

A phage-encoded enzyme called integrase inserts the viral DNA into a specific site within the bacterial chromosome, where it is now referred to as a prophage. In this state, the prophage becomes a passive passenger. As the bacterium lives and divides, it replicates its own DNA and, along with it, the integrated prophage, copying the viral genetic information into every new daughter cell.

The Prophage: A Latent Viral Presence

The integration of a prophage establishes a stable relationship between the phage and the bacterium, which is now termed a lysogen. This state is maintained by a repressor protein produced by the prophage itself. This protein circulates within the bacterial cytoplasm and shuts down the expression of most other phage genes, particularly those that would trigger the lytic cycle.

This same repressor protein confers a benefit known as superinfection immunity. Should another phage of the same type attempt to infect the lysogen, the repressor protein will prevent it from initiating an infection, protecting the host. The presence of a prophage can also alter the characteristics of the host bacterium in a phenomenon called lysogenic conversion. The prophage genome may carry genes that are expressed during latency and provide the bacterium with new traits, such as the production of toxins.

Induction: Awakening the Phage

A lysogen is not a permanent state, as the dormant prophage can be awakened to transition into the destructive lytic cycle. This switch is known as induction and is triggered by conditions that signal stress to the host bacterium, like significant DNA damage. Environmental factors such as UV radiation or exposure to certain chemicals can serve as triggers. The host’s own DNA repair system, the SOS response, often initiates this process. When extensive DNA damage is detected, a host protein interacts with the phage’s repressor protein, causing it to be inactivated.

With the repressor gone, the prophage genes are no longer silenced. The prophage excises itself from the bacterial chromosome, becoming an independent circle of DNA again. This event marks the beginning of the lytic cycle, where the now-active phage genome takes control of the host cell to replicate its DNA and assemble new phage particles. This process culminates in the lysis, or bursting, of the host cell, releasing the new phages.

Significance and Notable Examples of Lysogenic Phages

Lysogenic phages have profound implications for bacterial evolution and medicine. By integrating into and excising from bacterial chromosomes, they facilitate horizontal gene transfer, a process allowing bacteria to acquire new traits like antibiotic resistance. This mechanism is a significant driver of bacterial adaptation and the emergence of new pathogenic strains.

Many well-known bacterial diseases are caused by bacteria that are only harmful because they carry a prophage through lysogenic conversion. Notable examples include:

• The bacterium Vibrio cholerae becomes the agent of cholera only when infected by the CTXφ phage, which carries the genes for the cholera toxin.
• Corynebacterium diphtheriae causes diphtheria because a specific prophage provides it with the diphtheria toxin gene.
• The Lambda (λ) phage, which infects Escherichia coli, is a model system that has been instrumental in understanding the molecular details of lysogeny.
• Pathogenic strains of E. coli, such as EHEC, cause severe illness by harboring prophages that produce Shiga toxins.