Bacteriophages or phages (literally (Greek) – bacteria eater) – are viruses invading the bacteria. Like the rest of viruses phages are parasites – they cannot reproduce without a host cell. At that the phages are strictly specific: they define and invade only a definite kind of bacteria, sometimes – definite strains of some kind.

Bacteriophages form the most numerous and widely spread group of viruses on Earth.  Bacteria live everywhere, and the phages follow them all the time: in soil, in water, in plants, in the intestine of humans and animals, etc. The richer is the substrate in microorganisms, the more phages it contains. At the same time, they can exist in absence of the host microbe, remaining able to infect for decades. The bacteriophages are a crucial natural instrument for the microorganisms count control.

A typical phage particle consists of head and a tail. The head contains the genetic material (DNA or RNA). The tail provides interaction of the phage and the host bacterium. It resembles a hollow tube through which the genetic material gets from the phage head to the bacterium cytoplasm. On the end of the tail many phages have a basal plate with long thin threads helping the phage to fix on the surface of the bacterium.

By the nature of phages’ interaction with a bacterium they are divided into lytic and temperate. The first ones quickly kill (lyze) the host bacterium, whereas the second ones can inhabit the infected bacterium in a latent form.