In 1884 the Danish physician Christian Gram developed a new type of stain known today as the gram stain. He attempted to design a stain that differentiated bacterial cells from eukaryotic cells however he failed. What resulted from his work is one of the most important stains in microbiology. The Gram stain is an example of a differential stain. This type of stain takes advantage of the fact that different cells or different structures within cells react differently to various dyes. Gram positive bacteria retain the crystal violet-iodine complex after decoloring with alcohol or acetone and appear purple under the microsope. Gram negative bacteria lose the crystal violet-iodine complex and must be counter-stained with a red dye such as safranin. Gram negative bacteria appear pink under the microscope.
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