What type of microscopy is best for visualizing bacterial motility?

Dark-field microscopy is optimal for visualizing bacterial motility because it enhances the contrast of live, unstained bacteria against a dark background. This technique allows for the observation of the natural movement of microorganisms without the need for additional staining, which can kill the bacteria and interfere with motility. In dark-field microscopy, light is directed at an angle, causing only scattered light from the specimen to enter the lens, making it easier to see motile organisms such as spirilla or flagellated bacteria clearly.

While light microscopy can also visualize motility, it often requires the specimen to be stained or concentrated, which may obscure movement. Fluorescence microscopy is mainly used for identifying specific cellular components tagged with fluorescent dyes and is less effective for observing general motility. Electron microscopy, although providing high-resolution images of bacterial structures, does not permit the observation of live motility as it requires fixation and dehydration of samples. Thus, dark-field microscopy is the preferred method for observing bacterial motility directly.