Motile bacteria usually have one or more long, thin cellular appendages called .

Motile bacteria usually have one or more long, thin cellular appendages called flagella. Flagella serve as a means of locomotion, enabling bacteria to move toward or away from stimuli in their environment, a process known as taxis. The structure of flagella can vary among different bacterial species, but they generally consist of a filament, a hook, and a basal body.

The filament is the long, helical part that extends from the cell surface, the hook connects the filament to the basal body, and the basal body anchors the flagellum to the bacterial cell wall and membrane. Flagella are powered by a motor mechanism found in the basal body that utilizes the proton motive force generated across the bacterial membrane, allowing the bacteria to swim in liquid environments or to move across surfaces. The presence of flagella is critical for the survival and adaptation of many bacterial species in diverse environments.