Bacteriophage
Introduction
A bacteriophage, often shortened to phage, is a type of virus that infects and replicates within bacteria and archaea. The term "bacteriophage" is derived from "bacteria" and the Greek: φαγεῖν (phagein), "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 (e.g., MS2-like phages) and up to hundreds of genes. Phages replicate within the bacterium following the injection of their genome into its cytoplasm.
Biology
Bacteriophages are among the most common and diverse entities in the biosphere. Bacteriophages are ubiquitous viruses, found wherever bacteria exist. It is estimated there are more than 10^31 bacteriophages on the planet, more than every other organism on Earth, including bacteria, combined. One of the densest natural sources for phages and other viruses is sea water, where up to 9×10^8 virions per milliliter have been found in microbial mats at the surface, and up to 70% of marine bacteria may be infected by phages.
Classification
Bacteriophages occur abundantly in the biosphere, with different genomes, and lifestyles. Phages are classified by the International Committee on Taxonomy of Viruses (ICTV) according to morphology and nucleic acid.
Life Cycle
Two types of life cycles are seen in bacteriophages: the lytic cycle and the lysogenic cycle. The lytic cycle leads to the death of the host, whereas the lysogenic cycle leads to integration of phage into the host's genome.
Applications
Bacteriophages have been used for over 90 years as an alternative to antibiotics in the former Soviet Union and Central Europe, as well as in France. They are seen as a possible therapy against multi-drug-resistant strains of many bacteria.