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Biology LibreTexts

19.1: Learning Objectives

  • Page ID
    40274
  • Learning Objectives

    After this lab you should be able to:

    1. Perform a Gram Stain on a variety of bacterial cultures.
    2. Explain the principles of how the Gram Stain works.
    3. Evaluate and interpret your Gram Stain results correctly.
    4. Interpret and troubleshoot poor stain results.

    “A Gram stain of mixed Staphylococcus aureus (Staphylococcus aureus ATCC 25923, Gram-positive cocci, in purple) and Escherichia coli(Escherichia coli ATCC 11775, Gram-negative bacilli, in red), the most common Gram stain reference bacteria.” 1000x

    clipboard_e840b02ba26abb8331df65508ae6b1955.png
    Figure \(\PageIndex{1}\): Gram stain 01" by Y tambe - Y tambe's file. CC BY-SA 3.0

    https://commons.wikimedia.org/wiki/File:Gram_stain_01.jpg#/media/File:Gram_stain_01.jpg

    The stain you will perform in this lab was developed in 1884, yet is still the most commonly used stain in bacteriology, and one of the most important fundamental stains in diagnostic microbiology. In developing staining procedures using Crystal Violet, Christian Gram discovered that one procedure would differentiate bacteria into two groups. Some bacteria (no called Gram Positive) retain Crystal Violet throughout the procedure and stain purple. Others (now called Gram Negative) lose the Crystal Violet during the decolorizing step. These cells can be counterstained with a pink stain, Safranin. 1, 2

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