Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

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Simple Living Organisms
Flagella and Pili

Most Bacteria are motile, which is easily observed under a Microscope. Movement is driven by extremely thin, thread-like flagella with a diameter of 10–20 nm. Sometimes a bacterium has only a single flagellum, while in other cases There are two or more flagella located at one or both poles of The Cell; all flagella of a given bacterium move synchronously. In the corkscrew-shaped bacterium Spirillum (Fig. 1-2), a tuft of flagella at each end of the cell rotates at 50 revolutions per second, causing the entire cell to slowly rotate in the opposite direction and propel itself forward at speeds of up to 50 µm∙s^-1 [6]. Some strains of E. coli lack flagella altogether, whereas others have up to 8 per cell, distributed more or less randomly across the cell surface. When swimming, the Cells typically trail their flagella in a bundle behind them.

Another type of bacterial appendage remained entirely unsuspected until the advent of Electron Microscopy in this field. These are extremely fine (only 3 to 25 nm thick) long filaments known as pili (or fimbriae) (Fig. 1-2, A). Their function remains unknown in many cases, although it has been demonstrated that cells bearing a high density of these appendages readily adhere to one another. Certain pili (F-pili) in E. coli and related organisms apparently play a specific role in conjugation.