A phage (also called bacteriophage) (in Greek phageton = food/consumption) 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 Flix 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 Mller 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 chromosomal DNA of the host bacterium (much like endogenous retroviruses in animals) or establishing themselves as plasmids. These endogenous phages, referred to as prophages, are then copied with every cell division 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 λ of E. coli. Sometimes, prophages even provide benefit to the host bacterium while they are dormant, by adding new functions to the bacterial genome, a phenomenon called lysogenic conversion. A famous example is the harmless Vibrio 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 bacterial strains that are resistant to multiple drugs has led Western medical researchers to re-evaluate phages as alternatives to the use of antibiotics.
Following is a list of bacteriophages that are extensively studied: