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Unformatted text preview: Lab Exercise 8: The Gram Stain OBJECTIVES 1. Understand the chemical and theoretical basis for differential staining procedures, including the Gram Stain. 2. Perform a successful Gram Stain to differentiate between gram-positive and gram-negative bacteria. INTRODUCTION Christian Gram was a Danish physician of the 1800s who was trying to develop a staining procedure that would differentiate bacterial cells from eukaryotic ones in a stained tissue sample. Although he did not succeed in this endeavor, he did discover a monumental staining technique that distinguishes between different bacterial cell types based on the cell wall structure. In the Gram stain , the two kinds of cells, gram-positive and gram-negative , can be identified by their respective colors, purple and red/pink, after performing the staining method. Gram positive bacteria appear purple based on the fact that they retain a crystal violet-iodine complex through decolorization with alcohol or acetone. In contrast, alcohol or acetone removes the crystal violet-iodine complex from gram-negative bacteria. These bacteria must then be counterstained with a red dye, safranin, after the decolorization step to have any visible color (i.e....
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