Bacteriophage

Bacteriophage, 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 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).

Virulent Bacteriophages and the Lytic Cycle
Viruses that kill their infected host cell are called virulent.
 The DNA in these type of viruses reproduces through the lytic cycle.
 When these viruses reproduce, they break open, or lyse, their host cells,
 resulting in the destruction of the host. The whole cycle can be complete
 in 20 - 30 minutes depending on a variety of factors such as temperature.
 Phage reproduction is much faster than typical bacterial reproduction,
 so entire colonies can be destroyed very quickly.

Temperate Viruses and the Lysogenic Cycle
Temperate viruses are those that reproduce without killing their host cell.
 Typically they reproduce in two ways: through the lytic cycle and the lysogenic cycle.
 In the lysogenic cycle, the phage's DNA recombines with the bacterial chromosome.
 Once it has inserted itself, it is known as a prophage. A host cell that carries
a prophage has the potential to lyse, thus it is called a lysogenic cell. The image
 above illustrates both the lytic and lysogenic cycles of a bacteriophage.

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).