Why is Gram-Staining Important?

Gram-staining separates bacteria into gram-positive and gram-negative groups, based on the microorganisms' abilities to retain the dye crystal violet during de-colorization with alcohol.  The color difference between the two reflects differences in the outer surface structure of the microorganisms. In the picture to the right, the gram-negatives are the sections in light pink, and the gram-positives are in purple.

To Gram-Stain:

  • Rinse the slide with distilled water for a few seconds.

Interpreting the results:

So how do you tell which bacteria you observe under the microscope are gram-negative and which are gram positive? Gram-negative bacteria have a pink color because they lost the purple color of the crystal violet during the de-colorization process and were left with only the pink of the safranin. Gram-positive bacteria have a purple color since they retained the dye from the crystal violet, even after de-colorization.



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