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 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. 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 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.
Learn more about bacteriophage with a free trial on Britannica.com.
|
|
|
|
|
|
|
|
|
|