Flagellum
Flagellar arrangements

Different species of bacteria have different numbers and arrangements of flagella, named using the term tricho, from the Greek trichos meaning hair.

Counterclockwise rotation of a monotrichous polar flagellum pushes the cell forward with the flagellum trailing behind, much like a corkscrew moving inside cork. Water on the microscopic scale is highly viscous, unlike usual water.

Spirochetes, in contrast, have flagella called endoflagella arising from opposite poles of the cell, and are located within the periplasmic space as shown by breaking the outer-membrane and also by electron cryotomography microscopy. The rotation of the filaments relative to the cell body causes the entire bacterium to move forward in a corkscrew-like motion, even through material viscous enough to prevent the passage of normally flagellated bacteria.

In certain large forms of Selenomonas, more than 30 individual flagella are organized outside the cell body, helically twining about each other to form a thick structure (easily visible with the light microscope) called a " fascicle ".

In some Vibrio spp. (particularly Vibrio parahaemolyticus) and related bacteria such as Aeromonas, two flagellar systems co-exist, using different sets of genes and different ion gradients for energy. The polar flagella are constitutively expressed and provide motility in bulk fluid, while the lateral flagella are expressed when the polar flagella meet too much resistance to turn. These provide swarming motility on surfaces or in viscous fluids.
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