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lab_07_gram_stain - Miramar College Biology 205...

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Lab Exercise 7: The Gram Stain Page 1 of 3 Miramar College Biology 205 Microbiology Lab Exercise 7: The Gram Stain Background Christian Gram was a Danish physician in the 1800s who was trying to develop a staining procedure that would differentiate bacterial from eukaryotic cells in a stained tissue sample. Although he did not succeed in this endeavor, he did discover a monumental staining technique that distinguishes bacterial cell types based on cell wall structure. In the Gram stain , two kinds of cells, Gram positive and Gram negative , can be identified based on their ability to retain a primary stain throughout a staining process ( i.e. Gram positive) or their inability to do so ( i.e. Gram negative). Gram staining therefore divides the entire Domain of bacterial cells into two groups. Although Gram was unaware of it at the time, these differences in staining are caused by the amount of peptidoglycan in the cell wall. Gram positive cells appear purple because their thick layer of peptidoglycan will retain the primary stain, crystal violet, even when they are exposed to alcohol. Gram negative cells on the other hand have a very thin or absent peptidoglycan layer and so alcohol will completely remove all of the crystal violet from the cells (Figure 1). Figure 1: Micrograph of a Gram stain containing Gram positive cocci (purple) and Gram negative bacilli (pink) bacteria. With the Gram stain, you are being introduced to a new type of staining technique, differential staining . Differential staining is a method of subdividing microbes into two groups based on a single characteristic. In this case, as stated above, the presence of a thick peptidoglycan layer is differentiated. All methods of differential staining employ two stains, one to color the cells that are “positive” for a specific characteristic and one to color the cells that are “negative” for it.
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