bacteriophage

—bacteriophagic /bak tear'ee euh faj"ik, -fay"jik/, bacteriophagous /bak tear'ee of"euh geuhs/, adj. —bacteriophagy /bak tear'ee of"euh jee/, n.

/bak tear"ee euh fayj'/, n.

any of a group of viruses that infect specific bacteria, usually causing their disintegration or dissolution. Also called phage.

[1920-25; < F bactériophage. See BACTERIO-, -PHAGE]

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or phage

Any of a group of usually complex viruses that infect bacteria.

Discovered in the early 20th century, bacteriophages were used to treat human bacterial diseases such as bubonic plague and cholera but were not successful; they were abandoned with the advent of antibiotics in the 1940s. The rise of drug-resistant bacteria in the 1990s focused renewed attention on the therapeutic potential of bacteriophages. Thousands of varieties exist, each of which may infect only one or a few types of bacteria. The core of a bacteriophage's genetic material may be either DNA or RNA. On infecting a host cell, bacteriophages known as lytic or virulent phages release replicated viral particles by lysing (bursting) the host cell. Other types, known as lysogenic or temperate, integrate their nucleic acid into the host's chromosome to be replicated during cell division. During this time they are not virulent. The viral genome may later become active, initiating production of viral particles and destruction of the host cell. A.D. Hershey and Martha Chase used a bacteriophage in a famous 1952 experiment that supported the theory that DNA is the genetic material. Because bacteriophage genomes are small and because large quantities can be prepared in the laboratory, they are a favourite research tool of molecular biologists. Studies of phages have helped illuminate genetic recombination, nucleic acid replication, and protein synthesis.

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▪ virus

also called  phage,  or  bacterial virus,  

      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 (Hérelle, Félix d') in France (1917). D'Hérelle coined the term bacteriophage, meaning “bacteria eater,” to describe the agent's bacteriocidal ability.

      Thousands of varieties of phage exist, each of which may infect only one type or a few types of bacteria. Phages are classified in a number of virus families, including Inoviridae and Microviridae. 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.

      Phages have played an important role in laboratory research in the 20th century. The first phages studied were those designated type 1 (T1) to type 7 (T7). The T-even phages, T2, T4, and T6, were used as model systems for the study of virus multiplication. Alfred Day Hershey and Martha Chase used the T2 bacteriophage in a famous experiment in 1952 which supported the theory that DNA is the genetic material. Certain phages, such as lambda, Mu, and M13, are used in recombinant DNA work. The phage ϕX174 was the first organism to have its entire nucleotide sequence determined, a feat that was accomplished by Frederick Sanger (Sanger, Frederick) and colleagues in 1977.

      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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