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A stain used in bacteriology to distinguish between two physiologically distinct types of bacteria. The procedure involves staining the organisms with a basic dye, such as gentian violet or crystal violet, and adding a mordant, such as iodine or picric acid. In gram-positive bacteria these form a complex that cannot be removed by such decolorizing agents as acetone or alcohol. However gram-negative organisms lose the violet colour and on counterstaining with a red dye, such as carbol fuchsin or neutral red, will take up the red colour. Gram-positive organisms, on counterstaining, retain the original violet colour and can thus easily be distinguished from the red gram-negative bacteria. It is believed these differences may be related to cell wall structure. Thus gram-negative cell walls contain far more lipid than those of gram-positive cells, and a wider range of amino acids. However it is not clear whether it is the materials of the wall that act as the substrate for the differential staining reaction or whether they act by affecting the permeability of the wall. Gram-positive and gram-negative bacteria differ in many other ways apart from their staining reactions. For example, gram-positive bacteria are often more exacting in their nutritional needs and more susceptible to antibiotics but more resistant to plasmolysis. Examples of gram-negative bacteria are the Pseudo-monadales, Chlamydobacteriales, and various families of the Eubacteriales, e.g. Enterobacteriaceae and Brucellaceae. Gram-positive bacteria include those of the families Lactobacillaceae, Coryne-bacteriaceae, and Neisseriaceae.
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