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A phage (also called bacteriophage) is a small virus that infects only
bacteria. Like viruses, they consist of an outer protein hull and the
enclosed genetic material (which consists of dsDNA in 95% of the phages
known) of 20-650 kbp (kilo base pairs). Phages were discovered independently
by Frederick Twort in 1915 and by Flix DÕHerelle in 1917.
Phages infect only specific bacteria, and many are virulent phages, meaning
they reproduce within the infected bacterium, then lyse (destroy) it so the
new phages are released. (A famous quote from the microbiologist Mark Mller
says: Bacteria don't die, they just phage away.) Some phages (so-called
temperate phages), though, integrate their genetic material into the DNA of
the host bacterium and stay dormant, similar to endogenic retroviruses in
animals. These endogenic phages are then copied with every cell division
together with the DNA of the host cell. They do not harm 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
endogenic phages become active again and start their reproduction cycle,
resulting in the lysis of the host cell. For this reason, these phages have
also been called lysogenic phages. An example is phage λ in E. coli.
Sometimes, endogenic phages even benefit the host bacterium while they are
dormant by adding new functions to the bacterial genome. 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. They are also being evaluated as a medical
treatment against bacterial infections--because killing bacteria is what
phages do best.