Gram staining is a method for staining samples of bacteria that differentiates between the two main types of bacterial cell wall.
It is named after the inventor, the Danish scientist Hans Christian Gram (1853-1928), who developed the technique in 1884 to discriminate between pneumococci and Klebsiella pneumoniae bacteria.
Add iodinesolution (1% iodine, 2% potassium iodide in water]]) for 1 minute. This acts as a mordant and fixes the dye.
Rinse with water.
Apply 95% ethanol or a mixture of acetone and alcohol several times until no more colour appears to come from the sample.This leaves Gram-positive organisms stained purple and Gram-negative organisms unstained.
Rinse with water.
Apply a suitable counterstain. Opinions vary as to the best choice but suitable stains include safranin or fuchsin.This stains the gram negative organisms.
Gram positive organisms will appear blue-black or purple.
Gram negative organisms will appear red.
Gram-positive bacteria have a thick meshlike cell wall made of peptidoglycan which is capable of retaining the violet dye.
Gram negative bacteria have a thin cell wall made of a layer of peptidoglycan surrounded by an outer membrane containing lipids.
As a rule of thumb (which has exceptions), Gram-negative bacteria are more dangerous as disease organisms, because their outer membrane acts as "camouflage"; the human body does not contain peptidoglycan and in fact produces an enzyme called lysozyme which attacks the open peptidoglycan layer of Gram-positive bacteria. Gram-positive bacteria are also much more susceptible to penicillin.