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A bacteriophage (from 'bacteria' and Greek phagein, 'to eat') is any one of a number of viruses that infect bacteria. The term is commonly used in its shortened form, phage.

Typically, bacteriophages consist of an outer protein hull enclosing genetic material. The genetic material can be ssRNA (single stranded RNA), dsRNA, ssDNA, or dsDNA between 5 and 500 kilobase pairs long with either circular or linear arrangement. Bacteriophages are much smaller than the bacteria they destroy. Usually bacteriophage are between 20 and 200 nm in size, where as bacteria are 0.5 - 5 Ám, which is 500 - 5000 nm.

Phages are ubiquitous and can be found anywhere bacterial hosts exist, such as soil or the intestine of animals. They belong to the mobilome because they can transfer DNA between bacteria via transduction, a mechanism of horizontal gene transfer. One of the densest natural sources for phages and other viruses is sea water, where up to 109 virus particles per milliliter have been found at the surface, and up to 70% of marine bacteria may be infected by phages. They are also found in drinking water and in some foods, including fermented vegetables and meats, e.g. pickles, and salami, where they serve the function of controlling any growth of bacteria. - modified from wikipedia

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