Any of the viruses that infect bacterial cells. They are discrete particles with dimensions from about 20 to about 200 nanometers. A given bacterial virus can infect only one or a few related species of bacteria; these constitute its host range. Bacteriophages consist of two essential components: nucleic acid, in which genetic information is encoded (this may be either ribonucleic acid or deoxyribonucleic acid), and a protein coat (capsid), which serves as a protective shell containing the nucleic acid and is involved in the efficiency of infection and the host range of the virus.

The description of a bacterial virus involves a study of its shape and dimensions by electron microscopy , its host range, the serological properties of its capsid, the kind of nucleic acid it contains, and the characters of the plaques it forms on a given host. Both the nucleic acid and the capsid proteins are specific to the individual virus; in the case of the capsid proteins this specificity is the basis for serological identification of the virus.

The most striking form of phage infection is that in which all of the infected bacteria are destroyed in the process of the formation of new phage particles. This results in the clearing of a turbid liquid culture as the infected cells lyse. When lysis occurs in cells fixed as a lawn of bacteria growing on a solid medium, it produces holes, or areas of clearing, called plaques. These represent colonies of bacteriophage. The size and other properties of the plaque vary with individual viruses and host cells.Virus