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Chromophores: Groups with conjugated double bonds that give the dye its color.
Direct, cationic, basic or positive dyes: contain positively charged groups. Examples include methylene blue, basic fuchsin, and crystal violet. These dyes directly bind to and stain the negatively charged surface of bacterial cells.
Negative, anionic, or acidic dyes: contain functional groups that have a negative charge. Examples include eosin, nigrosin and Congo red. These dyes are repelled by the negatively charged surface of bacterial cells. Thus, they stain the background, leaving the bacterial cells clear and bright against a dark background.
Heat Fixation: application of heat to a bacterial smear preparation. This procedure simultaneously kills and attaches the bacteria to the slide.