bacteriophage, also called phage, or bacterial virus, any of a group of viruses that infect bacteria. Bacteriophages were discovered independently by Frederick W. Twortin Great Britain (1915) and Félix d’Hérelle in France (1917). D’Hérelle coined the term bacteriophage, meaning “bacteria eater,” to describe the agent’s bacteriocidal ability.
Thousands of varieties of phage exist, each of which may infect only one type or a few types of bacteria. Phages are classified in a number of virus families, including Inoviridae and Microviridae. Like all viruses, phages are simple organisms that consist of a core of genetic material (nucleic acid) surrounded by a protein capsid. The nucleic acid may be either DNA or RNA and may be double-stranded or single-stranded. There are three basic structural forms of phage: an icosahedral (twenty-sided) head with a tail, an icosahedral head without a tail, and a filamentous form.
A bacteriophage (informally, phage) is a virus that infects and replicates within bacteria. The term is derived from ‘bacteria’ and the Greek φαγεῖν phagein “to devour”. Bacteriophages are composed of proteins that encapsulatea DNA or RNA genome, and may have relatively simple or elaborate structures. Their genomes may encode as few as four genes, and as many as hundreds of genes. Phage replicate within bacteria following the injection of their genome into the cytoplasm. Bacteriophage are among the most common and diverse entities in the biosphere.
Phages are widely distributed in locations populated bybacterial hosts, such as soil or the intestines of animals. One of the densest natural sources for phages and other viruses is sea water, where up to 9×108 virions per milliliter have been found in microbial mats at the surface, and up to 70% of marine bacteria may be infected by phages. They have been used for over 90 years as an alternative to antibiotics in the former Soviet Union and Central Europe, as well as in France. They are seen as a possible therapy against multi-drug-resistant strains of many bacteria