T4 bacteriophage (DNA virus), animated coloured transmission electron micrograph (TEM). T4 is a bacteriophage that infects Escherichia coli bacteria. 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. Phages replicate within the bacterium following the injection of their genome into its cytoplasm. Bacteriophages are among the most common and diverse entities in the biosphere. Phages 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, where up to 1 trillion per millilitre have been found in microbial mats at the water surface, and up to 70% of marine bacteria may be infected by phages. Magnification: x55, 065 when shortest axis printed at 25 millimetres.