Bacteriophage

A bacteriophage, usually shortened to phage, is a virus that infects and multiplies inside bacteria. Many well-studied phages, such as T4, have a complex structure: a many-sided head, or capsid, that holds the genome, joined to a tail with fibers that grip specific receptors on the bacterial surface. Phages were discovered in the 1910s by Frederick Twort and Felix d'Herelle, and they are probably the most numerous biological entities on the planet.

After attaching, a phage injects its genome into the cell while its protein coat stays outside. A virulent phage then follows only the lytic cycle, taking over the cell to build new phages and finally bursting it open. A temperate phage, such as phage lambda, can instead enter the lysogenic cycle, in which its DNA joins the bacterial chromosome as a prophage and is copied along with it until stress switches it to the lytic cycle. Phages can also move bacterial genes from one cell to another, a process called transduction, and some carry toxin genes that make their hosts more dangerous.

Phages shape bacterial populations in soil, oceans and the gut. They are also being explored as phage therapy, a way to treat bacterial infections, including ones that resist antibiotics.