Coloured transmission electron micrograph (TEM) of bacteriophages (phage f2) on a pilus of the bacterium Escherichia coli. Bacteriophages are viruses which infect only bacteria, some specific to one species of bacterium. Phage f2 occurs only on male F-pili of E. coli. An F-pilus (containing the F plasmid) is a filament protruding from the wall of a male bacterium (bottom right) and is used in bacterial reproduction. Here, an F-pilus runs diagonally down the picture, with many RNA phage f2 (red spheres) attached to it. The red arc across the bottom of the picture is a curved bacterial flagellum. Magnification: x5100 at 6x4.5cm size.