Exam (elaborations) BIOD 171

Creating a hydrophobic barrier (the circle) helps to keep the water within the circle so it does not spill off of the slide. 4. What dye do Gram-positive bacteria primarily retain? Crystal violet 1. Why are Gram-positive bacteria able to retain the crystal violet dye? They contain a thick peptidoglycan layer in their cell wall that readily retains the dye. 2. Identify the Gram status (positive or negative) and shape of the bacteria pictured below. Gram-Negative; Bacillus (rod) 3. An acid-fast stain is most commonly used to identify what type of bacterium? What is the name of the primary dye used in this technique? Mycobacterium; carbolfuchsin dye 4. Why do bacteria repel the dye nigrosin? Nigrosin is a negatively charged dye. The membranes of most cells are also negatively charged. The membrane will repel the dye not allowing it to be absorbed. 1. What is one disadvantage of heat fixing a sample? The heat fixing procedure kills the specimen. This also prevents any observations on motility and enzymatic properties.