6.5 Peritrichous Flagella Atlas of Bacterial and Archaeal Cell Structure Home
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Swarming

Proteus mirabilis adapt their motility machinery to their environment. In liquid, the short rod-shaped cells swim with the help of a handful of flagella distributed peritrichously around their cell body. When they encounter a solid surface, the cells elongate and build many more flagella, as you can see on this cell. Instead of swimming, they now use their flagella to propel themselves in groups across the surface, a motility mode known as “swarming.” This is an example of a differentiated lifecycle, which will come up again in Chapter 8.

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Peritrichous Flagella

Still other species are peritrichous (“hair around”), with multiple flagella distributed randomly around the cell, as you can see on this Pseudomonas flexibilis. The number of flagella varies between different species, from relatively few here to considerably more (⇩). The well-known model system Escherichia coli is also peritrichously flagellated. In this arrangement, when the flagellar motors are all rotating one direction (counter-clockwise), the flagella form a whip-like bundle that propels the cell to “run” in a straight line. When one or more motors switch to clockwise rotation, the flagella dissociate from the bundle and “tumble” the cell to face a new direction. In the next chapter, you will see how cells use this behavior to seek out favorable spots.

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