A prophage is the genetic material of a bacteriophage—a virus that infects bacteria—after it has been incorporated into a host bacterium’s chromosome. Alternatively, it can exist as a separate, self-replicating plasmid within the cell. In this state, the viral DNA remains dormant, integrated into the host’s life cycle.
How Prophages Are Formed
A prophage is formed through the lysogenic cycle. This process begins when a bacteriophage attaches to a bacterium and injects its genetic material into the host cell. Following this injection, the phage can either enter the destructive lytic cycle or the more passive lysogenic cycle.
Opting for the lysogenic path leads to prophage formation. The phage’s DNA, instead of taking over the cell for immediate virus production, integrates itself directly into the bacterial chromosome. This integration often occurs at specific attachment sites, and the viral genes are silenced, becoming a quiet passenger in the bacterial genome.
In some cases, the phage’s genetic material forms an independent, circular piece of DNA called a plasmid instead of integrating into the chromosome. This plasmid is still considered a prophage because it remains dormant and is replicated along with the host’s DNA. This ensures its persistence without immediate harm to the cell.
This integration allows the virus to propagate itself every time the bacterium divides, without having to produce new viral particles. The phage DNA hitches a ride, its own identity temporarily submerged within the larger genetic blueprint of the bacterium.
The Dormant Life of a Prophage
Once established, the prophage enters a dormant state maintained by repressor proteins produced by its own genes. These proteins bind to the phage DNA, blocking the expression of genes required for viral replication and host cell destruction.
During this inactivity, the bacterium treats the prophage’s genetic material as its own. It is replicated every time the cell divides, so each daughter cell inherits a copy of the prophage, passing the viral genome through generations.
This stable relationship between the prophage and its host can persist for long periods if the bacterium remains in a healthy environment. This state, known as lysogeny, allows the phage to multiply its genome without killing its host, an effective strategy for its long-term survival.
Prophage Activation: Return of the Phage
The dormant state of a prophage is not permanent and can be reversed through a process called induction. This activation triggers the prophage to exit the bacterial chromosome and enter the lytic cycle, resulting in new phage particles and the death of the host cell. Induction is initiated by stress signals that indicate the host bacterium is in distress.
A primary trigger for induction is DNA damage to the host bacterium. Exposure to agents like ultraviolet (UV) radiation or certain chemicals can damage the bacterial chromosome. This damage activates a cellular stress response, which inactivates the repressor proteins that keep the prophage dormant.
Once the repressor proteins are no longer functional, the prophage excises itself from the bacterial chromosome. The active phage genome then takes control of the host cell’s resources to replicate viral DNA and synthesize new phage proteins. These components assemble into hundreds of new phage particles.
The final step is the lysis, or bursting, of the bacterial cell, which releases the newly formed phages into the environment to infect other bacteria. This switch from a dormant prophage to an active, destructive virus demonstrates the dynamic nature of the phage-bacterium relationship.
The Influence of Prophages on Their Bacterial Hosts
Prophages can alter the characteristics of their bacterial hosts through a phenomenon known as lysogenic conversion. These changes can transform a harmless bacterium into a pathogen or provide it with survival advantages. This happens because the host cell expresses genes carried by the prophage, conferring novel traits.
A well-known example is toxin production. The bacterium Corynebacterium diphtheriae only causes diphtheria when it carries a prophage containing the gene for the diphtheria toxin. Similarly, Vibrio cholerae owes its ability to cause cholera to a prophage that provides the instructions for the cholera toxin, and the Shiga toxin from Escherichia coli O157:H7 is also encoded by prophage genes.
Beyond toxin production, prophages can grant their hosts resistance to certain antibiotics. They can also provide immunity against infection by other, related phages, a benefit known as superinfection exclusion. Some prophages carry genes that modify the bacterial cell surface, affecting the bacterium’s ability to form biofilms or adhere to surfaces.
These modifications contribute to the evolution and genetic diversity of bacterial populations. By introducing new genes, prophages act as agents of horizontal gene transfer. This process shuffles genetic information between bacteria, allowing them to adapt to new environments and impacting their evolution and virulence.