Microbiolists classify bacteria into two groups: gram-positive or gram-negative. A Gram Stain is done with a crystal violet dye. Gram-positive bacteria have a thick cell wall and thus will take up the crystal violet dye and turn a purple color because they retain the dye.

Gram-negative bacteria will take up the crystal violet dye but will not retain it because they have a very thin cell wall. As a result gram-negative bacteria will turn a pink color.

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