Flagella are structures that function in cell motility in a liquid medium. In bacteria flagella are long and thin, attached to the cell at one end. Flagella attach to cells at different locations. Various types of flagella connect in different sites of a cell. For example, polar flagella connect at one or both ends of a cell. Another type of flagella, peritrichous flagella, flagella insert at many locations around the cell. Flagellation type is used to characterize bacteria.

The flagella of bacteria are so thin that they cannot be seen with light microscopy unless stained but the electron microscope can see it. During a type of polar flagellation, a group of flagella may rise at one end of a cell. This groups, called a tuft, can be seen by dark field or phase contrast microscopy.

Flagella are not straight but helical and anchored to the cytoplasmic membrane and cell wall. In bacteria, the flagella are made of flagellin, a protein. Flagellin plays a part in shape and wavelength of the flagellum.  The highly conserved amino acid sequence in flagellin suggests that motility evolved early in bacteria.

The flagellum is a rotary motor. In gram-negative cells, the rotor is made up of a central rod that passes through the L ring, P ring, and the MS and C rings. These rings and the central rod comprises the basal body.  These rings are anchored to the LPS layer, peptidoglycan layer, cytoplasmic membrane and the cytoplasm respectively. The second component of a rotary motor, the stator, consisted of Mot proteins that surround the basal body and generate torque.

The energy required to rotate the flagellum come from the proton motive force. The movement of protons across the cytoplasmic membrane and through the Mot complex driven the rotation of the flagellum. About 1000 protons are translocated per a rotating basis.