A bacteriophage injects its genetic material into a bacterial cell. In most phages, that genetic material is double-stranded DNA, though some phages carry single-stranded DNA, and others carry RNA. The protein shell that houses the genome stays outside the bacterium. Nothing else gets delivered: the entire point of injection is to get the phage’s genetic instructions inside the cell so they can hijack its machinery.
Types of Genetic Material Phages Carry
The majority of well-studied phages package double-stranded DNA inside their protein head (called a capsid). The two largest families used in research and diagnostics, Myoviridae and Siphoviridae, both carry double-stranded DNA. But phages are diverse. Filamentous phages like M13 have a long, rod-shaped body wrapped around a single-stranded DNA genome. Still others use RNA, either single-stranded or double-stranded, as their genetic blueprint.
Regardless of the type, the genetic material contains everything the phage needs to reproduce: genes encoding its structural proteins, enzymes for copying its genome, and instructions for assembling new phage particles. Once inside the bacterium, these genes get read by the cell’s own protein-building machinery, effectively turning the bacterium into a phage factory.
How the Injection Works
Phage injection isn’t passive. It’s a multi-step mechanical process, best understood in tailed phages like T4, which infects E. coli.
First, the phage has to find and grab onto the right bacterium. Long tail fibers extending from the phage’s base recognize specific molecules on the bacterial surface. For T4, those targets are sugar-containing molecules called lipopolysaccharides and a membrane protein called OmpC. The tail fiber tips fit into these surface molecules the way a key fits a lock, which is why each phage species can only infect certain bacteria.
Once at least three long tail fibers have latched on, they tug on the phage’s baseplate, triggering a cascade of shape changes. Short tail fibers then unfold and lock onto the cell surface permanently. This irreversible attachment is the point of no return.
Next, the tail sheath contracts like a spring-loaded syringe, driving a rigid inner tube through the bacterium’s outer membrane. At the tip of this tube sits a spike protein with a needle-like structure that physically punctures the membrane. That same spike protein contains a built-in enzyme that breaks down peptidoglycan, the tough mesh layer in the bacterial cell wall. Once the tube has punched through both the outer membrane and the cell wall, the genome flows through the hollow tube directly into the cell’s interior.
What Drives the DNA Through
The genome doesn’t just drift in. Inside the phage capsid, DNA is packed incredibly tightly, coiled under enormous pressure. Measurements of phage lambda show internal capsid pressures between 20 and 60 atmospheres, far higher than the few atmospheres of pressure inside a bacterial cell. That pressure difference acts like a compressed spring being released, forcefully pushing the DNA through the tail tube and into the host.
A typical phage genome, stretched out, is around 10 micrometers long, roughly ten times the length of the bacterial cell receiving it. The full transfer takes anywhere from a few seconds to several minutes. The speed depends on factors like genome size, the viscosity of the surrounding environment, and how much resistance the DNA encounters as it threads through the narrow channel.
The Protein Coat Stays Behind
This is a key detail that sometimes gets overlooked. The capsid, tail, tail fibers, and baseplate all remain stuck to the outside of the bacterium after injection. Only the nucleic acid enters. This was famously demonstrated in the 1952 Hershey-Chase experiment, which used radioactive labels to show that phage DNA, not protein, entered bacterial cells during infection. That experiment helped confirm DNA as the molecule carrying genetic information.
What Happens After Injection
Once the genetic material is inside, one of two things typically happens, depending on the phage and the conditions.
In the lytic pathway, the phage genes immediately take over. The bacterial cell starts copying the phage genome, building new capsids, tails, and fibers, then assembling complete phage particles. Eventually the cell bursts open, releasing dozens to hundreds of new phages that go on to infect neighboring bacteria.
In the lysogenic pathway, the phage DNA quietly integrates itself into the bacterium’s own chromosome. It gets copied every time the bacterium divides, passed along to daughter cells without causing harm. The phage essentially goes dormant. In phage lambda, this decision hinges on a molecular tug-of-war between two regulatory proteins encoded in the injected DNA. One protein locks the phage into dormancy by silencing its replication genes. The other promotes active replication. The balance between these two proteins, influenced by the cell’s stress level and nutrient availability, determines which path wins. If conditions later become stressful (UV damage, starvation), the dormant phage can reactivate, switch to the lytic pathway, and destroy the host cell.
Some phages are strictly lytic and always destroy their host. Others, called temperate phages, can choose either path. But in every case, the process starts the same way: the phage injects its genetic material, and that nucleic acid alone is enough to commandeer the entire cell.