Molecular model of a pilus machine from a bacterium. A pilus is a hair-like protein filament found on the surface of a bacterial cell. They are used for locomotion and to adhere to substrates. The pilus machine is a molecular machine that spans the bacterial cell wall. A rotary motor on the cytoplasmic (inner) side of the cell wall adds new subunits to the growing pilus, which is feed through a collar to the outer membrane.