Bacteria, single-celled organisms, exist alongside viruses called bacteriophages. These viruses engage in intricate interactions with bacteria, often influencing their behavior and survival in profound ways. This interplay reveals complex biological mechanisms that shape microbial communities across virtually every environment on Earth.
Bacteriophages and Their Life Cycles
Bacteriophages, often called phages, are viruses that specifically infect bacteria. Each phage is composed of genetic material, either DNA or RNA, encased within a protein shell. When a phage encounters a bacterial cell, it attaches to the surface and injects its genetic material into the host, initiating one of two primary life cycles: lytic or lysogenic.
In the lytic cycle, the phage takes over the host cell’s machinery to produce many new phage particles. The viral DNA remains separate from the bacterial DNA, directing the synthesis of viral components. This process culminates in the lysis of the bacterial cell, releasing the newly formed phages to infect other cells.
Alternatively, some phages can enter a lysogenic cycle. Instead of immediately replicating, the phage’s genetic material integrates into the bacterial chromosome, becoming a prophage. In this state, the phage DNA is replicated along with the host’s DNA each time the bacterial cell divides, without harming the host. This allows the phage to persist within the bacterial population across many generations.
How Prophages Form
A prophage represents the integrated genetic material of a bacteriophage within the DNA of a bacterial host. This unique state is a defining characteristic of the lysogenic cycle. When a temperate phage infects a bacterium, its DNA does not immediately commandeer the cell’s resources for viral replication.
Instead, specific enzymes encoded by the phage facilitate a process of recombination, seamlessly inserting the phage DNA into the bacterial chromosome. This integration means the viral genome becomes a stable part of the bacterial cell’s own genetic information. In this integrated form, the prophage is dormant; its genes responsible for producing new phage particles are largely inactive.
As the bacterial cell grows and divides, the prophage DNA is replicated along with the bacterial chromosome, ensuring that every daughter cell inherits a copy of the integrated phage genome. This allows the phage to propagate without killing its host, essentially hitchhiking within the bacterial lineage.
When Prophages Become Active
While a prophage can remain dormant within a bacterial chromosome for many generations, it can become activated, transitioning from the lysogenic state back into the lytic cycle. This activation, known as prophage induction, is often triggered by environmental stress or DNA damage to the host bacterium. Common triggers include exposure to ultraviolet (UV) radiation, certain chemicals, or some antibiotics that cause DNA damage.
Upon induction, the prophage DNA excises, or detaches, itself from the bacterial chromosome. Once freed, the phage genetic material reactivates its genes, initiating the replication process to produce new viral components. These components then assemble into new phage particles within the host cell. The cell ultimately lyses, releasing new phages into the environment.
Impact of Prophages on Bacteria and Beyond
Prophages profoundly influence bacterial evolution and diversity through a process known as lysogenic conversion, also called phage conversion. This occurs when the integrated prophage carries genes that confer new traits to its bacterial host. These acquired traits can alter the bacterium’s characteristics, impacting its survival and interaction with its environment.
A notable example of lysogenic conversion is the acquisition of virulence factors, which are traits that enhance a bacterium’s ability to cause disease. For instance, the bacteria responsible for diphtheria, botulism, cholera, and certain types of food poisoning can produce toxins only when they carry specific prophages. These toxins are encoded by genes within the prophage, turning a harmless bacterium into a pathogen. Prophages can also transfer genes related to antibiotic resistance, contributing to the spread of drug-resistant bacteria. This horizontal gene transfer, mediated by phages, allows bacteria to rapidly adapt to changing conditions and acquire beneficial genes from other bacteria.