The first external structure is the pilus (plural: pili). A pilus is a thin, rigid fiber made of protein that protrudes from the cell surface. The primary function of pili are to attach a bacterial cell to specific surfaces or to other cells. Pili can also aid in attachment between bacterial cells.

A pilus (plural: pili) is a hair-like appendage found on the surface of many bacteria. All pili are primarily composed of pilin proteins, which are oligomeric. Dozens of these structures can exist on the bacteria. Some bacteria, viruses or bacteriophages attach to receptors on pili at the start of their reproductive cycle. Pili are antigenic. They are also fragile and constantly replaced, sometimes with pili of different composition, resulting in altered antigenicity.