19.1: Learning Objectives
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After this lab you should be able to:
- Perform a Gram Stain on a variety of bacterial cultures.
- Explain the principles of how the Gram Stain works.
- Evaluate and interpret your Gram Stain results correctly.
- 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
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
Contributors and Attributions
Kelly C. Burke (College of the Canyons)