Hanging Drop

Hanging Drop, a method for observing microorganisms in their living state, first introduced by Koch, serves for studying the motility of microorganisms, the nature of their reproduction and spore germination, their relationship to chemical irritants (tropism reactions), the effect on them of immune sera (agglutination and lysis reactions), etc. To prepare a hanging drop, one needs: 1) clean slides with a depression in the center ('well slide'), 2) clean cover slips, 3) vaseline, 4) a platinum loop, 5) a burner, 6) a fresh (for bacteria-not older than 24-hour) culture of the microorganism being studied in liquid or solid nutrient media, and 7) physiological (0.85%) NaCl solution.

1-hanging drop; 2-well slide; 3-object slide; 4-cover slip.

Procedure for preparing a hanging drop: 1. A drop of not too thick a suspension of microorganisms in liquid nutrient medium or in 0.85% NaCl is applied with a platinum loop, previously heated over a burner flame and cooled, to the middle of a cover slip lying on the laboratory table. 2. The cover slip with the applied drop of microbial suspension is tightly covered with an object slide, the edge of whose well is smeared with vaseline. The object slide should be placed on the cover slip so that the well is above the drop. Thanks to the vaseline, the cover stick adheres to the object slide, and the drop is in a hermetically sealed space. 3. By careful and quick movement, the preparation is turned over so that the cover slip with the drop is on top, and the object slide with the well is on the bottom (see the figure). 4. The drop is examined using a dry or oil immersion microscope system. For initial orientation under the microscope, the edge of the hanging drop is found first, and then the drop is studied throughout its entire length. Microscopic examination of the hanging drop must be done with slightly darkened field of view (with the illuminator lowered or with the iris diaphragm narrowed). For special purposes and prolonged observations, special chambers exist that allow maintaining a constant temperature in them and protect the hanging drop from drying out. The hanging drop method is also often used in the study of yeasts, algae, protozoa, etc.