A set of two stains that are used to stain (crystal violet and iodine) and counterstain (fuchsin) bacteria. gram-negative bacteria are completely decolorized after treating the stained bacteria with ethanol or acetone and can be visualized by the counterstain. They have a complex cell wall in which the peptidoglycan layer is covered by lipopolysaccharides in an outer membrane structure. gram-positive bacteria resist decolorization by ethanol or acetone, they retain the initial gram stain. They have a cell wall consisting predominantly of peptidoglycan not overlayed by an outer membrane.
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 155 (https://doi.org/10.1351/pac199264010143)