Bacteriophage have different three-dimensional shapes (or morphologies). Those that are known as T-even phages (i.e., T2, T4, and T6) have a shape similar to the Apollo spacecraft that landed on the Moon in the 1960s. These phages have a head that has a slightly spherical shape called an icosahedron. A tube connects the head to spider-like supporting legs. This overall shape is vital to the way T-even bacteriophage deliver their payload of genetic material into a bacterial cell. Once on the surface on a bacterium, the tube portion of the phage contracts, and the phage acts like microscopic hypodermic needles, literally injecting the genetic material into the bacterium.
Other bacteriophage can be spherical (e.g. PhiX174, S13) or long and thread-like (e.g., M13).
Structurally, bacteriophage, like most viruses, are composed of a protein coat surrounding a core containing DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), though many variations on this basic design exist. The bacteria these viruses infect often measure about one micron in diameter (a micron is one thousandth of a millimeter) and the phages themselves may be as small as twenty-five thousandths of a micron. Bacteriophage infect their hosts by binding to the wall of the bacterial cell. The wall is perforated by enzyme action and phage DNA is injected into the cell. The genetic machinery of the cell is altered to make more bacteriophage DNA. Ultimately, the host cell dies when phage copies accumulate to the point of lysing (bursting) the cell membrane and releasing the phages, which go forth and continue the cycle.