A Bacteriophage
is a virus that infects bacteria and sometimes destroys them by lysis,
or dissolution of the cell. Bacteriophages, or phages, have a head
composed of protein, an inner core of nucleic acid (either
deoxyribonucleic acid (DNA) or ribonucleic acid [RNA])) and a hollow
protein tail. A particular phage can usually infect only one or a few
related species of bacteria.
A phage infects a bacterial cell
by first attaching to the bacterial cell wall by its tail. The tail is
generally a complex protein structure consisting of a hollow
contractile sheath, with a plate at the base that contains long
protein fibers. The tail fibers fix the base plate to the specific
receptor site on the bacterial cell wall, and the tail sheath
contracts like a syringe, forcing the DNA that is inside the virus
through the cell wall and cell membrane. The entire virus protein coat
remains outside the bacterium.
The injected nucleic acid is the
viral genetic material; it makes use of the bacterium's chemical
energy and biosynthetic machinery to produce viral enzymes, as well as
more phage nucleic acid. The viral proteins and nucleic acid molecules
within the bacterial host assemble spontaneously into up to a hundred
new phage particles. Eventually the bacterium lyses, releasing the
particles. Lysis can be readily observed in bacteria growing on a
solid medium, where groups of lysed cells appear as clear areas, or
plaques.
Some DNA phages, called temperate
phages, only lyse a small fraction of bacterial cells; in the
remaining majority of the bacteria, the phage DNA becomes integrated
into the bacterial chromosome and replicates along with it. In this
state, known as lysogeny, the information contained in the viral
nucleic acid is not expressed. A lysogenic bacterial culture can be
treated with radiation or mutagens, inducing the cells to begin
producing viruses and lyse. Lysogenic phages resemble bacterial
genetic particles known as episomes. Incorporated phage genes are
sometimes the source of the virulence of disease-causing bacteria.
The bacteriophage was discovered
independently by the microbiologists F. W. Twort (1915) and Félix
d'Hérelle (1917). The phages have been much used in the study of
bacterial genetics and cellular control mechanisms largely because the
bacterial hosts are so easily grown and infected
with phage in the laboratory. Phages were also used in an
attempt to destroy bacteria that cause epidemic diseases, but this
approach was largely abandoned in the 1940s when antibacterial drugs
became available. The possibility of "phage therapy" has recently
attracted new interest among medical researchers, however, owing to
the increasing threat posed by drug-resistant bacteria.
Bacteriophage:
Also called Phage or Bacterial Viruses,
bacteriophage are any of a group of viruses that infect bacteria.
Bacteriophages were discovered independently by Frederick W. Twort in
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. 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.
During infection a phage attaches
to a bacterium and inserts its genetic material into the cell. After
this a phage follows one of two life cycles, lytic (virulent) or
lysogenic (temperate). Lytic phages take over the machinery of the
cell to make phage components. They then destroy, or lyse, the cell,
releasing new phage particles. Lysogenic phages incorporate their
nucleic acid into the chromosome of the host cell and replicate with
it as a unit without destroying the cell. Under certain conditions
lysogenic phages can be induced to follow a lytic cycle.
Soon after making their discovery,
Twort and d'Hérelle began to use phages in treating human bacterial
diseases such as bubonic plague and cholera. Phage therapy was not
successful, and after the discovery of antibiotics in the 1940s it was
virtually abandoned. With the rise of drug-resistant bacteria in the
1990s, however, the therapeutic potential of phages has received
renewed attention.
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