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A phage (also called bacteriophage) is a small virus that infects only bacteria. Like viruses that infect eukaryotes, phages consist of an outer protein hull and the enclosed genetic material (which consists of double-stranded DNA in 95% of the phages known) of 5 to 650 kbp (kilo base pairs) with a length of 24 to 200 nm. The vast majority of phages (95%) have a tail to let them inject their genetic material into the host. Phages were discovered independently by Frederick Twort in 1915 and by Félix d’Herelle in 1917. d'Herelle continued his research and development in Stalin's Soviet Union.

Phages infect only specific bacteria. Some phages are virulent, meaning that upon infecting a cell they immediately begin reproducing, and within a short time lyse (destroy) the cell, releasing new phages. (A famous quote from the microbiologist Mark Müller says: Bacteria don't die, they just phage away.) Some phages (so-called temperate phages) can instead enter a relatively harmless state, either integrating their genetic material into the chromosom al DNA of the host bacterium (much like endogenous retrovirusAlpharetrovirus Betaretrovirus Gammaretrovirus Deltaretrovirus Epsilonretrovirus Lentivirus Spumavirus A retrovirus is a virus which has a genome consisting of RNA. It relies on reverse transcriptase to perform a kind of reverse transcription of its genomes in animals) or establishing themselves as plasmidPlasmids are (typically) circular double stranded DNA molecules that are separate from the chromosomal DNA (Fig. They usually occur in bacteria, sometimes in eukaryotic organisms (e. the 2-micrometre-ring in Saccharomyces cerevisiae . Their size varies frs. These endogenous phages, referred to as prophages, are then copied with every cell divisionCell division is the process of a biological cell (called a mother cell dividing into two daughter cells''. This leads to growth in multicellular organisms (the growth of tissue) and to procreation ( vegetative reproduction) in unicellular organisms. together with the DNA of the host cell. They do not kill the cell, but monitor (via some proteins they code for) the status of their host. When the host cell shows signs of stress (meaning it might be about to die soon), the endogenous phages become active again and start their reproductive cycle, resulting in the lysis of the host cell. An example is phage λEnterobacteria phage &lambda lambda phage is a temperate phage that lives in E. Once the phage is inside its host, it may integrate itself into the host's DNA. In this state, λ is called a prophage and stays resident within the host's genome, witho of E. coli. Sometimes, prophages even provide benefit to the host bacterium while they are dormant, by adding new functions to the bacterial genomeGenome is also a popular science book by Matt Ridley. In biology, the genome of an organism is a complete DNA sequence of one set of chromosomes; for example, one of the two sets that a diploid individual carries in every somatic cell. When people say tha, a phenomenon called lysogenic conversionLysogenic conversion is when a temperate phage induces a change in the phenotype of the bacteria infected that is not part of a usual phage cycle. Changes can often involve the external membrane of the cell by making it impervious to other phages or even. A famous example is the harmless VibrioVibrio cholerae Vibrio fischeri Vibrio harveyi Vibrio vulnificus Vibrio parahaemolyticus Vibrio is a genus of bacteria, a member of the gamma group of the Proteobacteria, normally associated with diarrheal disease and other symptoms. Species include: Vibr bacteria strain, which is turned into Vibrio cholerae by a phage, causing cholera.

Phages play an important role in molecular biology as cloning vectors to insert DNA into bacteria. Phage therapy has been used since the 1940s in the former Soviet Union as an alternative to antibiotics for treating bacterial infections—because killing bacteria is what phages do best. There is an extensive library of research into specific phages and their therapeutic uses in the Tbilisi Institute in Georgia. The development of multiply resistant bacterial strains has led Western medical researchers to re-evaluate phages as alternatives to the use of antibiotics.