Microbiology => Gram's Stain
Gram's Stain, widely used method of staining bacteria as an aid to their identification, devised by Hans Christian Joachim Gram, a Danish physician. In Gram's method, bacteria are first stained with gentian violet (a dye consisting of a methyl derivative of pararosaniline) and then treated with Gram's solution, consisting of 1 part iodine, 2 parts potassium iodide, and 300 parts water. After being washed with ethyl alcohol, the bacteria will either retain the strong blue color of gentian violet or be completely decolorized. Sometimes a counterstain such as fuchsine or eosin is applied to give the decolorized bacteria a reddish color to make them more visible.
Bacteria that retain the blue stain are known as gram-positive; those that do not are known as gram-negative. Organisms that sometimes retain the blue color and sometimes do not are known as gram-variable. Typical gram-positive bacteria are those staphylococci that produce boils; typical gram-negative bacteria are the bacilli that cause whooping cough; typical gram-variable bacteria are the bacilli that cause tuberculosis.