Transmission Electron Micrograph (TEM) of Bacteriophage T4. Phage viruses are those causing lysis in their bacterial cell hosts. T4 is a large bacteriophage which infects Escherichia coli., a natural inhabitant of the human gut. The physiology of such viruses has been extensively researched. The viral head resembles an elongated icosahedron, containing the DNA genome. The tail is composed of a collar joining it the head, a central hollow tube with surrounding sheath and a complex base plate. Tail fibers (seen here) are responsible for virus attachment to the proper site on the bacterial cell surface. Magnification: 165,000.