The hanging drop technique is a special type of wet mount, often used to determine whether organisms are motile. Smears of appropriate thickness are allowed to air-dry completely and are then passed through an open flame. This process, called heat fixation, kills the organisms, causing them to adhere to the slide and more readily accept stains. B. Principles of Staining A stain, or dye, is a molecule that can bind to a structure and give it color. MOST bacterial surfaces are negatively charged so they will attract the basic dyes.
Simple stains use one dye and reveal basic cell shapes and arrangements.
Differential stains use two or more dyes and distinguish various properties or organisms. The Gram stain , spore stain, and acid-fast stain are examples. Negative stains color the background around cells and their parts, which resist taking up the stain. Please see Figures 3.28-31 and Table 3.3 for more details on the different types of staining techniques. We will also be covering these in great detail in the laboratory.