Bacteriophages, also called phages, are bacterial viruses that invade bacterial cells. Like all viruses, phages are metabolically inert and reproduce themselves only after infecting suitable host bacteria. Phages can have either a lytic or a lysogenic life cycle. In case of lytic phages, they disrupt bacterial metabolism and cause bacteria to lyse. Bacteriophages are often highly specific and non toxic to animals and plants.

Coloured transmission electron micrograph of a T4 bacteriophage (orange) after injecting its viral DNA into the Escherichia coli bacterium (blue). Bacteriophages are viruses that infect bacteria; T4 is specific to E.coli. It is secured to the surface of the cell by spidery tail fibres. The tail, a contractile sheath, shortens to allow a syringe-like tube (below base plate of tail) to enter the cell membrane, emptying the DNA contents of the head into the bacterium. The capsid, a protein coat surrounding the DNA, stays outside. Phage' synthesis in the cell follows rapidly. Magnification: X65,000 at 6x7cm size. magnification: x220,000 at 10x8 size.