When the chemical stimulus is an attractant, such as a rich nutrient source, the basal body is made to rotate so that the bacteria swim in straight lines toward the signal for long periods of time. Flagella can be visualized by a somewhat difficult staining technique, but their presence is typically based on detecting motility of living bacteria. Observations can be made in a hanging drop and using semisolid motility agar. This method is commonly used to view living organisms for the rapid determination of motility. The hanging drop is prepared by suspending a fluid sample from a coverslip over a depression well in a specially designed microscope slide.
Wet mounts can be used for the same purpose, however, wet mounts tend to dehydrate rapidly.
Hanging drops, on the other hand, are sealed within the depression and retain their liquid for longer periods of time. In both methods, the living specimen is unstained. For best results, reduce the amount of light passing through the specimen. With the toothpick, put a small amount of petroleum jelly on each corner of the coverslip. This will help the coverslip adhere to the slide.