Flagella

Many bacteria are motile. This ability to move independently is usually attributed to a special structure, the flagella (singular: flagellum). A flagellum is a threadlike appendage extending outward from the plasma membrane and cell wall. Flagella are slender, rigid, locomotor structures, about 20 pm across and up to 15 to 20 pm long. Depending on the species, a cell may have a single flagellum (i.e., monotrichous bacteria; trichous means "hair") (Figure 8.3); one flagellum at each end (i.e., amphitrichous bacteria; amphi means "on both sides"); a tuft of flagella at one or both ends (i.e., lophotrichous bacteria; lopho means "tuft"); or flagella that arise all over the cell surface (i.e., peritrichous bacteria; peri means "around").

Flagellation patterns are very useful in identifying bacteria and can be seen by light microscopy, but only after the flagella have been stained using special techniques designed to increase their thickness. The detailed structure of flagella can be seen only in the electron microscope.

Bacterial cells benefit from flagella in several ways. They can increase the concentration of nutrients or decrease the concentration of toxic materials near the bacterial surfaces by causing a change in the flow rate of fluids. They can also disperse flagellated organisms to areas where colony formation can take place. The main benefit of flagella to organisms is that they improve the organism's ability to flee from areas that might be harmful.

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