Use of the Gram stain in microbiology

The Gram stain differentiates bacteria into two fundamental varieties of ce lls. Bacteria that retain the initial crystal violet stain (purple) are sai d to be "Gram-positive," whereas those that are decolorized and stain red w ith carbol fuchsin (or safranin) are said to be "Gram-negative." This stain ing response is based on the chemical and structural makeup of the cell wal ls of both varieties of bacteria. Gram-positives have a thick, relatively i mpermeable wall that resists decolorization and is composed of pepticloglyc an and secondary polymers. Gram-negatives have a thin peptidoglycan layer p lus an overlying lipid-protein bilayer known as the outer membrane, which c an be disrupted by decolorization. Some bacteria have walls of intermediate structure and, although they are officially classified as Gram-positives b ecause of their linage, they stain in a variable manner. One prokaryote dom ain, the Archaea, have such variability of wall structure that the Gram sta in is not a useful differentiating tool.