31.13: Motility Test
- Page ID
- 165424
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The motility test is used to determine whether a bacterial organism is capable of self-directed movement, typically through the use of flagella. Motility is a valuable characteristic in the identification of unknown bacterial isolates, particularly among members of the Enterobacteriaceae family and other rod-shaped bacteria. Motility agar provides a more reliable alternative for detecting bacterial motility. The medium contains a reduced agar concentration (usually around 0.4 percent), which creates a soft consistency that allows motile bacteria to move through the medium. Non-motile bacteria will remain restricted to the line of inoculation. To enhance visibility of growth, the redox-sensitive dye tetrazolium salt is included. This indicator turns bright pink or red in areas where bacterial metabolism has occurred, helping to clearly show the pattern of movement.
Procedure
- Inoculate the medium by stabbing the sterile needle straight down through the center of the agar to approximately two-thirds of the tube’s depth
- Withdraw the needle along the same stab line, being careful not to disturb the surface
- Incubate at 30°Celsius for 48 hours
Interpretation of Results
Hold the tube up towards the light then observe the growth pattern in the tube
- Diffuse, hazy growth that extends outward from the stab line (often with pink or red coloration if an indicator is used) indicates a positive result for motility
- Growth restricted to the stab line, with no lateral spread, indicates a negative result for motility
- If an indicator is present, color change throughout the medium enhances visibility of the result but is not required to confirm motility