Which technique is used to visualize motility in bacteria?

The hanging drop method is a technique used to directly observe bacterial motility under a microscope. This method involves placing a drop of liquid culture containing bacteria on a microscope slide, which is inverted over a depression slide. This setup creates a chamber that allows for observation of live bacteria in their natural configuration and state of motility.

By using the hanging drop method, one can see how bacteria move, whether they are motile or non-motile, through the use of natural light microscopy. The movement can be observed without the interference of a solid medium, allowing for a clear view of individual motile cells and their movement patterns, such as swimming or darting.

Other methods mentioned do not effectively visualize motility in the same manner. The double agar method also involves the use of semi-solid media but is not as effective for real-time observation of motility. The spread plate and pour plate methods are primarily used for isolating and quantifying bacteria rather than for observing motility; these methods involve solid media which may inhibit the natural movement of bacteria.