Many bacteria have flagella that make them capable of swimming through water-based environments. Motility can be determined by observing cells in a wet mount. However this determination can be difficult because bacteria are small enough that they are bounced around by water molecules. This random movement, called Brownian motion, can be confused with self-propelled motility.
Another way to determine motility is with a Motility Agar stab. Motility Agar is soft agar in a test tube (without a slanted surface). Cells are stab-inoculated into the agar (the top surface is not inoculated). Non-motile bacteria will only grow where they were inoculated. Motile bacteria will grow along the stab and will also swim out away from the stabbed area. Thus, a negative result is indicated by growth in a distinct zone directly along the stab. A positive result is indicated by diffuse (cloudy growth), especially at the top and bottom of the stab.