title: Use of the gram stain in microbiology.
abstract: The Gram stain differentiates bacteria into two fundamental varieties of cells. Bacteria that retain the initial crystal violet stain (purple) are said to be "gram-positive," whereas those that are decolorized and stain red with carbol fuchsin (or safranin) are said to be "gram-negative." This staining response is based on the chemical and structural makeup of the cell walls of both varieties of bacteria. Gram-positives have a thick, relatively impermeable wall that resists decolorization and is composed of peptidoglycan and secondary polymers. Gram-negatives have a thin peptidoglycan layer plus an overlying lipid-protein bilayer known as the outer membrane, which can be disrupted by decolorization. Some bacteria have walls of intermediate structure and, although they are officially classified as gram-positives because of their linage, they stain in a variable manner. One prokaryote domain, the Archaea, have such variability of wall structure that the Gram stain is not a useful differentiating tool.
authors.affiliation: Department of Microbiology, College of Biological Science, University of Guelph, Ontario, Canada. tjb@micro.uoguelph.ca
authors.forename: T J
authors.initials: TJ
authors.lastname: Beveridge
mesh_terms: D001419:Bacteria / Q000145:classification / Q000302:isolation & purification*; D005840:Gentian Violet; D008829:Microbiology / Q000639:trends; D010619:Phenazines
chemical_list: C035863:Gram's stain; D010619:Phenazines; D005840:Gentian Violet