Some motile bacteria possess structures which help them to swim in their habitat. Motility is most common among the spirilla and bacilli but lacking or very rare among the coccal forms.
The organs of locomotion are called flagella (sing flagellum). These are cytoplasm in nature. Each flagellum arises from the cytoplasm membrane. A granule called blepharoplast is present very close to it and helps it to pass out through the cell wall.
The number, position and arrangement of flagella vary with the type of bacteria. These may be restricted to one or both ends of the bacterium cell. This type of flagellation is called polar. It may also be distributed all over the body surface (no polar flagellation)
When the bacterium cell bears single flagellum inserted at one pole only, it is called monolrichous. When the flagellum occurs at each pole, the bacterium is amphitrichous.
Two or more flagella together occur in a bunch at one pole of the bacterium cell, it is cephalotrichous type. Bacteria possessing two or more flagella at both the poles are of lophotrichous type. When flagella are uniformly distributed on the bacterial cell, it is of peritrichous type. Bacteria with no flagellation, at all, are called atrichous.
Main component of flagellum is a protein called flagellin. Each flagellum is composed of a single thin fibril.