Most viruses of bacteria, or bacteriophages, use a tail structure for the controlled injection of their genome and internal proteins into the bacterial host. The tail, together with proteins located in the capsid of the phage, translocates DNA and proteins from the capsid into the cytoplasm of the host cell while crossing the wall and cell membrane in gram-positive bacteria or two cell membranes and the peptidoglycan layer in gram-negative bacteria. Yet another function of the tail is to coordinate host recognition, which is performed by tail fiber and tailspike proteins emanating from the tail, with irreversible attachment to the host’s surface.