Phage

Phage or Bacteriophage is a type of virus that infects and replicates within bacteria and archaea. The term "phage" is derived from the Greek word phagein, which means "to devour". Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have relatively simple or elaborate structures. Their genomes may encode as few as four genes, and as many as hundreds of genes.

History

The discovery of bacteriophages dates back to the early 20th century. In 1915, British bacteriologist Frederick Twort first discovered bacteriophages, and Felix d'Herelle, a French-Canadian microbiologist, independently confirmed this discovery in 1917. D'Herelle quickly recognized the potential of phage for treating bacterial infections, which led to the development of phage therapy.

Structure

Bacteriophages are generally composed of an outer protein hull enclosing genetic material. The genetic material can be either DNA or RNA, and can be double-stranded or single-stranded. The simplest phages just have an icosahedral (20-sided) head/bacteriophage capsid enclosing their genetic material, and a tail structure that can attach to the host bacteria.

Life Cycle

Bacteriophages may have a lytic cycle or a lysogenic cycle, and a few viruses are capable of carrying out both. In the lytic cycle, the bacteriophage makes more of itself by commandeering the bacterium's machinery. The bacteriophage then causes the bacterium to burst, or lyse. In the lysogenic cycle, bacteriophages are more patient. Instead of killing the host, the phage genome integrates into the bacterial chromosome and becomes part of the host.

Applications

Bacteriophages have been used for over 90 years as an alternative to antibiotics in the former Soviet Union and Eastern Europe. They are seen as a possible therapy against multi-drug-resistant strains of many bacteria. Phages are currently being used therapeutically to treat bacterial infections that do not respond to conventional antibiotics, particularly in Russia and Georgia.

See Also

• Virus
• Bacteria
• Phage therapy
• Frederick Twort
• Felix d'Herelle

Phage Resources
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