practical exercises

Hanging drop preparation


Some bacteria are motile and some other are non-motile. Motile bacteria usually use flagella as their locomotory organ. Bacteria tend to move towards or away from various chemotactic, phototactic, aerotactic or magnetotactic stimuli. Description of bacterial flagella is available here. There are various ways to demonstrate bacterial motility. These include, a simple wet mount, hanging drop preparation, or employment of soft gels (semi-solid agar).

Requirements for hanging drop preparation: Fresh broth culture of bacteria, bunsen burner/spirit lamp, bacteriological loop, glass slide (with central concavity/paraffin ring/adhesive ring), cover slips and microscope.

Procedure: A loopful of bacterial suspension is placed in the center of a cover slip. The glass side with stick ring is placed over the coverslip such a way that coverslip sticks to the ring on the slide. Immediately, the glass slide is lifted and turned around. The drop of bacterial suspension now "hangs" on the lower surface of the coverslip. The drop is then observed under the low power (10x) dry objective of the compound microscope. The edge of the drop must be focused. Bacteria tend to accumulate at the edge of the drop. Once the edge is located, it is then observed under 40x high power objective.

Observation under 10x objective:

hanging drop
Edge of the drop is seen.

Observation under 40x objective:

hanging drop
Motile bacilli are seen at the edge of the drop.