A bacteriophage is a virus that infects and replicates within a bacterium. It is composed of proteins that encapsulate a DNA or RNA genome. Bacteriophages (‘phages’) have relatively simple structures and their genomes encodes something between four and several hundred genes. To replicate, phages inject their genome into the cytoplasm of a bacteria.

Bacteriophages are among the most common and diverse entities in the biosphere. They are widely distributed in locations populated by bacterial hosts, such as soil or the intestines of animals. One of the densest natural sources for phages and other viruses is sea water.

This hand colored scanning-electron-micrograph shows bacteria (orange) which are heavily infected by bacteriophages (blue, white).

At the lower right hand side, a bacterial cell has been broken open (lysed) and destroyed. As soon as the cell is destroyed, the phage progeny can find new hosts to infect.