Pili are molecular hair-like projections found on the surface of cells of many Gram- positive and Gram-negative species. They are composed of molecules of a protein called pilinarranged to form a tube with a minute, hollow core. There are two general classes, common pili and sex pili (Fig 2 – 12). Common pili cover the surface of the cell. They are, in many cases, adhesins, which are responsible for the ability of bacteria to colonize surfaces and cells. To cite only one example, the pili of Neisseria gonorrhoeaeare necessary for the attachment to the urethral epithelial cells prior to penetration; with- out pili, the bacterium cannot cause gonorrhea. Thus, common pili are often important virulence factors. In fact, there are at least five different types of common pili (see Chap- ter 10). Some bacteriologists use the name fimbriae to refer to common pili. The sex pilus is diagnostic of a male bacterium and is involved in exchange of genetic material between some Gram-negative bacteria. There is only one per cell.