Bacterial motility constitutes unicellular behavior. In other words, motile bacteria are capable of a behavior called taxis. Taxis is a motile response to an environmental stimulus and functions to keep bacteria in an optimum environment. The arrangement of the flagella about the bacterium is of use in classification and identification. One group of bacteria, the spirochetes , has internally-located axial filaments or endoflagella.
Axial filaments wrap around the spirochete towards the middle from both ends.
They are located above the peptidoglycan cell wall but underneath the outer membrane or sheath. Direct observation of motility using special-purpose microscopes. The special optics convert slight variations in specimen thickness into corresponding visible variation in brightness. Thus, the bacterium and its structures appear darker than the background . A dark-field microscope uses a special condenser to direct light away from the objective lens.