Flagella and Pili

Many bacteria swim at speeds of 20–60 µm/s, ten or more body lengths per second! Very thin threadlike flagella of diameter 13–20 nm coiled into a helical form are rotated by the world’s smallest “electric motors” to provide the motion. 14 While some bacteria
have a single flagellum, the corkscrew-like Spirillum (Fig. 1-3) synchronously moves tufts of flagella at both ends. Some strains of E. coli have no flagella, but others contain as many as eight flagella per cell distributed over the surface. The flagella stream out behind in a bundle when the bacterium swims. The flagella of the helical spirochetes are located inside the outer membrane. In addition to flagella, extremely thin, long, straight filaments known as pili or fimbriae project from the surfaces of many bacteria. 14 The “sex pili” (F pili and I pili) of E. coli have a specific role in sexual conjugation. The similar but more numerous common pili or fimbriae range in thickness from 3 to 25 nm and in length from 0.2 to 2 m. Pili are involved in adhesion of bacteria to surrounding materials or to other bacteria and facilitate bacterial infections. A typical E. coli cell has 100–300 pili.