Which method is described as the most accurate for detecting motility in nonfermentative bacteria?

• A. Semisolid motility medium
• B. Hanging-drop method
• C. Wet mount
• D. Gram stain

Answer: (B)

Observing motility directly in a living suspension is the clearest way to tell if bacteria can move. The hanging-drop method places a small drop of broth or saline between a slide and a coverslip, then inverts it so the drop is suspended and sealed. This setup lets you watch individual cells swim in real time under phase-contrast or other microscopy, giving you direct evidence of motility, the speed and patterns of movement, and whether flagella are driving that motion. Because the cells remain in their natural state with minimal disturbance, this method minimizes artifacts and provides the most reliable indication of true motility for nonfermentative bacteria.

In contrast, semisolid motility media infer motility from how far cells spread from the inoculation point, which can be influenced by growth rate, diffusion, or swarming and may not accurately reflect true motility. Wet mounts are useful for quick checks but can dry out or alter conditions during observation, introducing artifacts. Gram staining, on the other hand, reveals cell morphology and Gram reaction but not movement, so it cannot determine motility.