Characterisation by motility testing: Hanging drop method
Many bacteria show no motion and are termed nonmotile. However, in an aqueous environment, these same bacteria appear to be moving erratically. This erratic movement is due to Brownian movement. Brownian movement results from the random motion of the water molecules bombarding the bacteria and causing them to move.
True motility (self-propulsion) has been recognized in other bacteria and involves several different mechanisms. Bacteria that possess flagella exhibit flagellar motion. Helical-shaped spirochetes have axial fibrils (modified flagella that wrap around the bacterium) that form axial filaments. These spirochetes move in a corkscrew- and bending-type motion. Other bacteria simply slide over moist surfaces in a form of gliding motion.
The above types of motility or nonmotility can be observed over a long period in a hanging drop slide. Hanging drop slides are also useful in observing the general shape of living bacteria and the arrangement of bacterial cells when they associate together. A ring of Vaseline around the edge of the coverslip keeps the slide from drying out.