Refers to the inability of a type of bacterium to resist decolorization with alcohol after being treated with crystal violet. However, following decolorization, these bacteria can be readily counterstained with safranin, imparting a pink or red color to them when viewed by light microscopy. This reaction is usually an indication that the outer structure of the bacterium consists of a cytoplasmic (inner) membrane surrounded by a relatively thin peptidoglycan layer, which in turn is surrounded by an outer membrane.
See: Gram stain
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Examples: glitazone, GI cocktail, etc.