Gram Negative and Gram Positive

The Gram stain, named after Danish bacteriologist Christian Gram who developed the technique, is a method of dividing 99% of bacteria into two distinct categories. When bacteria are stained with certain basic dyes (e.g. crystal violet) the cells of some species can easily be decolorized with solvents such as ethanol or acetone and are counter stained pink. These are labeled Gram-negative bacteria. Cells that resist decolorization remain purple and are called Gram-positive. The ability to retain or lose the stain generally reflects fundamental structural differences in the cell wall. As the cell wall is one of the prime targets for antimicrobial activity, many antimicrobials will be much more effective in one category than the other. (e.g., penicillin better with Gram positives.)