3Tues_Negative stains

3Tues_Negative stains - Nigrosin dye Staining tray Sterile...

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Lab Exercise 7: The negative stain OBJECTIVES 1. Perform a negative staining procedure 2. Understand the benefits of using negative stains. INTRODUCTION As mentioned in lab exercise 6, not all dyes stain the bacterial cell. However, acidic dyes can be useful for observing the size and shape of bacterial cells and for cells such as spirochaetes that do not readily stain with ordinary dyes. In these negative stains, the background area around the cell appears dark or opaque. Cells appear as transparent against this dark background, and this allows you to observe such extracellular features as capsules. Negative stains are also not heat fixed, to avoid shrinkage of the cells and destruction of the capsule. LAB EXERCISES Table supplies Individual supplies Cultures of Bacillus megaterium Microscope slides Cultures of Staphylococcus aureus
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Unformatted text preview: Nigrosin dye Staining tray Sterile toothpicks Protocol: 1. Place one drop of nigrosin onto the center of a clean slide. 2. Using sterile technique, transfer one loopful of B. megaterium to the drop of nigrosin and mix. 3. Spread the mixture evenly on the slide to at least the size of a quarter. 4. Repeat steps 2 and 3 using S. aureus . 5. Repeat steps 2 and 3 using a plaque sample from between teeth, obtained with a sterile toothpick. 6. Let the preparations air-dry. DO NOT heat fix. 7. Examine all stained slides under 100x oil immersion lens. DATA AND OBSERVATIONS 1. Draw observations of cells from your plaque sample and B. megaterium . Plaque sample Total magnification ___________ Total magnification ___________ DISCUSSION 1. Why are negative stains useful? 2. Do you heat fix a negative stain? Why?...
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3Tues_Negative stains - Nigrosin dye Staining tray Sterile...

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