Non-motile bacteria are bacteria that show no motion. They lack the ability to swim toward optimal areas for survival, and they don't swim away from toxicity or toward nutrients and optimal light concentrations. This purposeful, spontaneous form of movement that most bacteria engage in is nonexistent in this form of bacteria.
Some non-motile bacteria may appear to move due to the random motion of surrounding water molecules. This type of motion is called Brownian motion and does not prove motility of the bacteria. Non-motile bacteria can't move because they lack the appendages necessary for movement.
Many motile bacteria have thread-like appendages that extend out from the cell wall and allow them to move. These appendages are called flagellum. Flagellum act like a boat propeller, moving the organism away from unfavorable environments. In some organisms, contractions of cytoplasm help to produce a form of movement called gliding. Organisms called spirochetes have axial filaments that wind around the organism and attach to its poles, providing it with the ability to move.
The range of motility and the number and distribution of flagella in bacteria are important characteristics that are used to identify and classify them. Flagella can either be polar, arising from one or both ends of the cell, or peritrichous, arranged randomly over the entire surface.