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(băktēr`ēəfāj'), virusvirus,
parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes.
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 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 acidnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
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—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; for example, coliphages are DNA-containing viruses that infect only the bacterium Escherichia coli.

A virus infects a bacterial cell by first attaching to the bacterial cell wall by its tail. In coliphages the tail is 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 episomesepisome
, unit of genetic material composed of a series of genes that sometimes has an independent existence in a host cell and at other times is integrated into a chromosome of the cell, replicating itself along with the chromosome. Episomes have been studied in bacteria.
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. 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. In 2006 the Food and Drug Administration approved the use of bacteriophages that attack strains of Listeria as a food additive on ready-to-eat meat products.


Any of the viruses that infect bacterial cells. They are discrete particles with dimensions from about 20 to about 200 nanometers. A given bacterial virus can infect only one or a few related species of bacteria; these constitute its host range. Bacteriophages consist of two essential components: nucleic acid, in which genetic information is encoded (this may be either ribonucleic acid or deoxyribonucleic acid), and a protein coat (capsid), which serves as a protective shell containing the nucleic acid and is involved in the efficiency of infection and the host range of the virus.

The description of a bacterial virus involves a study of its shape and dimensions by electron microscopy (see illustration), its host range, the serological properties of its capsid, the kind of nucleic acid it contains, and the characters of the plaques it forms on a given host. Both the nucleic acid and the capsid proteins are specific to the individual virus; in the case of the capsid proteins this specificity is the basis for serological identification of the virus.

Diagram of a T4 bacteriophageenlarge picture
Diagram of a T4 bacteriophage

The most striking form of phage infection is that in which all of the infected bacteria are destroyed in the process of the formation of new phage particles. This results in the clearing of a turbid liquid culture as the infected cells lyse. When lysis occurs in cells fixed as a lawn of bacteria growing on a solid medium, it produces holes, or areas of clearing, called plaques. These represent colonies of bacteriophage. The size and other properties of the plaque vary with individual viruses and host cells. See Lysogeny, Lytic infection, Virus


Any of the viruses that infect bacterial cells; each has a narrow host range. Also known as phage.
References in periodicals archive ?
After immunization, rabbit antisera have demonstrated a specific activity towards M13 bacteriophages.
M13 bacteriophage has the ability to generate electricity when it is compressed but lacks the toxicity of the traditional elements.
The combination of MALDI-MS and the Sanger sequencing method has been used for M13 bacteriophage DNA, with sequence being determined for DNA as much as 35 bases long [12].