Temperate bacteriophages are the type of phage that can enter an inactive prophage stage. Unlike strictly lytic phages, which immediately hijack a bacterial cell and destroy it to release new virus particles, temperate phages have a choice. They can either reproduce and kill the host cell or quietly insert their genetic material into the bacterial chromosome, becoming a prophage that sits dormant and replicates along with the bacterium every time it divides.
This ability to “go quiet” is the defining feature of temperate phages and the basis of what microbiologists call the lysogenic cycle. Understanding how this works reveals a surprising amount about bacterial disease, antibiotic resistance, and even the composition of your own microbiome.
Two Life Cycles, One Virus
Every temperate phage faces a fork in the road after infecting a bacterial cell. In the lytic cycle, the phage takes over the cell’s machinery, copies itself hundreds of times, and bursts the cell open to release new virus particles. In the lysogenic cycle, the phage does something radically different: it integrates its DNA into the host bacterium’s chromosome and stops producing new virus particles entirely. The integrated phage DNA is now called a prophage.
A prophage is not dead or broken. It is simply silent. Each time the bacterium copies its own DNA and divides, it copies the prophage right along with it. Every daughter cell inherits the prophage. A single infection event can seed an entire bacterial population with viral DNA, all without producing a single new virus particle. This can continue for thousands of bacterial generations.
How the Phage Decides: Lytic or Lysogenic
The decision between the lytic and lysogenic pathways is controlled by a molecular switch, best studied in a phage called lambda that infects E. coli. Two key proteins compete for control. One protein, called CI, acts as a repressor that keeps all the virus’s reproduction genes turned off, pushing the phage toward lysogeny. The other, called Cro, blocks CI production and pushes toward lytic replication. These two proteins form a double negative feedback loop: CI represses Cro, and Cro represses CI. Whichever protein gains the upper hand first locks the phage into that pathway.
When CI wins, it not only shuts down the lytic genes but also activates its own production, creating a self-reinforcing loop that keeps the prophage silent. This makes lysogeny remarkably stable. The prophage can remain inactive indefinitely, passed from one bacterial generation to the next, as long as nothing disrupts that repressor.
Environmental conditions nudge the decision. When a bacterium is healthy and nutrients are abundant, the lysogenic pathway is more likely. When conditions are poor, the phage is more likely to go lytic, essentially abandoning a sinking ship by making copies of itself before the host cell dies on its own.
How a Prophage Wakes Up
A dormant prophage can reactivate through a process called prophage induction. The most well-understood trigger is DNA damage. When a bacterial cell’s DNA is damaged by UV light, certain chemicals, or antibiotics like mitomycin C, the cell activates an emergency DNA repair system called the SOS response. A protein called RecA, part of this emergency system, promotes the destruction of the CI repressor that was keeping the prophage quiet. Once CI is gone, Cro takes over, the lytic genes switch on, and the phage cuts itself out of the chromosome and begins producing new virus particles. The bacterial cell is destroyed in the process.
This is why prophage induction matters clinically. Antibiotics or other stressors that damage bacterial DNA can inadvertently trigger dormant phages to wake up, potentially releasing toxins or spreading viral genes to new bacteria.
How the Phage DNA Gets In
Integration requires a specialized enzyme called an integrase, encoded by the temperate phage itself. The integrase recognizes two specific DNA sequences: one on the phage genome and one on the bacterial chromosome. It cuts both sequences and swaps the strands, stitching the phage DNA seamlessly into the bacterial chromosome.
Integrases fall into two major families. One type, exemplified by lambda phage, recognizes longer DNA sequences and needs help from bacterial proteins to complete the job. The other type is larger, recognizes shorter sequences, and works independently. In the case of lambda phage, integration happens at a specific spot on the E. coli chromosome, located at a well-characterized position between two genes. Researchers have experimentally moved this landing site to other locations on the chromosome and shown that integration still works, confirming that the integrase-attachment site pairing is the key requirement, not the surrounding genes.
Why Prophages Matter for Human Disease
Prophages are not just hitchhikers. They often carry genes that change how bacteria behave, a phenomenon called lysogenic conversion. Some of the most dangerous bacterial toxins in medicine are encoded not by the bacterium’s own genes but by prophages hiding in its chromosome. Diphtheria toxin, cholera toxin, botulinum toxin, Shiga toxin, and pertussis toxin are all prophage-encoded. As early as 1951, researchers demonstrated that diphtheria toxin production depended entirely on the presence of a prophage. Remove the prophage, and the bacterium becomes harmless.
This means that a non-pathogenic bacterium can become dangerous simply by acquiring a temperate phage carrying a toxin gene. Shiga toxin-producing E. coli, responsible for serious foodborne illness, is a direct example: the toxin gene sits on a prophage.
How Much Bacterial DNA Is Actually Viral
Prophage sequences are remarkably common. Studies of the human microbiome show that prophage DNA makes up an average of 1 to 5 percent of each bacterial genome. That may sound small, but across the trillions of bacteria in the human body, it represents an enormous reservoir of viral genetic material.
Prophages also serve as vehicles for horizontal gene transfer between bacteria. When a prophage excises imprecisely during induction, it can accidentally package neighboring bacterial genes and carry them to a new host cell. This process, called transduction, can spread antibiotic resistance genes, metabolic capabilities, or virulence factors between bacterial species. Three forms of transduction exist: specialized (carrying genes adjacent to the integration site), generalized (carrying random bacterial DNA), and lateral (transferring large segments of the host chromosome). Each represents a different way prophages shuffle genetic material through bacterial communities.
Why Phage Therapy Avoids Temperate Phages
Phage therapy, the use of viruses to treat bacterial infections, generally relies on strictly lytic phages rather than temperate ones. The reasoning is straightforward. A temperate phage might integrate into the target bacterium instead of killing it, failing as a therapeutic. Worse, it could deliver virulence genes or antibiotic resistance genes to the very bacteria you’re trying to eliminate. Because most known bacterial virulence genes are carried by temperate phages, using them therapeutically poses a real risk of making an infection more dangerous rather than less.
Researchers have explored engineering temperate phages to remove their ability to integrate, locking them into a lytic-only mode. But for now, the standard practice in phage therapy is to screen candidates carefully and use only obligately lytic phages that lack the genetic machinery for lysogeny.