Bacteriophages attacking Streptococcus bacteria. Coloured scanning electron micrograph (SEM) of A25 bacteriophages (yellow) attacking Streptococcus pyogenes bacteria. A bacteriophage, or phage, is a virus that infects bacteria. A25 infects Streptococcus pyogenes bacteria. It consists of an icosahedral (20-sided) head, which contains the genetic material, a tail (cylindrical) and a transverse plate with a spike. The spike attaches to the surface of a bacterium and the tail injects a DNA (deoxyribonucleic acid) strand into the cell. The viral genetic material then hijacks the bacterium's own cellular machinery, forcing it to produce more copies of the bacteriophage. When a sufficient number have been produced, the phages burst out of the cell, killing it.