A prophage is the genetic material of a bacteriophage, a virus that infects bacteria, integrated into the bacterial host’s genome. This integrated viral DNA becomes a stable part of the bacterium’s genetic makeup, allowing the phage to exist within the bacterium without immediately causing harm.
Bacteriophages and Their Life Cycles
Bacteriophages, often simply called phages, are viruses that target and infect bacteria. These viruses possess distinct life cycles that determine their interaction with the bacterial host. The two main strategies are the lytic cycle and the lysogenic cycle.
In the lytic cycle, the phage injects its genetic material into the bacterial cell, taking over the host’s machinery to produce many new phage particles. This culminates in the lysis, or bursting, of the bacterial cell, releasing the newly formed phages to infect other cells.
In contrast, temperate phages can undergo a lysogenic cycle, where the phage’s genetic material integrates into the host bacterial DNA, forming a prophage. Instead of destroying the cell, the prophage’s DNA is replicated along with the bacterial DNA as the host cell divides. This allows the phage to reproduce without immediately harming its host.
The Prophage State and Its Characteristics
The prophage state involves bacteriophage DNA stably integrated into the bacterial chromosome. This integration is facilitated by specific enzymes, such as integrases, encoded by the phage itself. The prophage DNA then replicates passively alongside the bacterial DNA during cell division, ensuring its propagation to daughter cells.
During this dormant state, most of the prophage’s genes are repressed, and new phage particles are not produced. The bacterium carrying a prophage is referred to as a lysogen. This stable relationship allows the phage to persist within the bacterial population across many generations.
Impact of Prophages on Bacterial Hosts
Prophages can significantly alter the characteristics and behavior of their bacterial hosts, a phenomenon known as lysogenic conversion. This process can confer new traits to bacteria, influencing their survival and interaction with their environment.
Prophages can lead to the acquisition of virulence factors, enhancing a bacterium’s ability to cause disease. For example, Vibrio cholerae produces cholera toxin only when infected by the CTXφ phage, and Corynebacterium diphtheriae produces diphtheria toxin when it carries the phage β. Staphylococcus aureus can acquire the Panton-Valentine leukocidin toxin, contributing to necrotizing pneumonia and skin infections, through prophage integration.
Prophages also play a role in transferring genes that contribute to antibiotic resistance among bacteria. They act as vectors for horizontal gene transfer, allowing bacteria to acquire new resistance determinants. This mechanism can contribute to the spread of antibiotic resistance, posing challenges for treating bacterial infections.
Beyond virulence and resistance, prophages contribute to the genetic diversity of bacterial populations, influencing bacterial evolution and adaptation. The presence of a prophage can also provide the bacterial host with immunity to superinfection by other phages of a similar type. This superinfection exclusion mechanism prevents other phages from successfully infecting the same cell, protecting the resident prophage and the host bacterium from lysis.
Activation and Release of New Phages
A prophage can transition from its dormant state back into the lytic cycle, a process termed induction. This shift is often triggered by various environmental stressors that indicate unfavorable conditions for the bacterial host.
Common triggers for prophage induction include DNA damage from exposure to ultraviolet (UV) radiation or certain chemicals like mitomycin C and fluoroquinolones. These stressors activate bacterial repair mechanisms, such as the SOS response, leading to the excision of the prophage DNA from the bacterial chromosome. Once excised, the prophage DNA replicates rapidly, leading to the assembly of new phage particles. This culminates in the lysis of the bacterial cell, releasing the progeny phages to infect new hosts.