Gram staining

Method of staining bacteria developed by Hans Christian Joachim Gram in 1884. For this, the bacteria are stained with alkaline crystal violet for 1 - 3 minutes. After decantation of the staining solution, the bacteria are treated with iodine-potassium iodide producing a water-insoluble complex. Then, this solution is decanted as well. Afterwards, the bacteria are rinsed with alcohol. Results: Gram-positive bacteria retain the blue dye, Gram-negative release it. Gram did not carry out counterstaining with basic fuchsin or safranin as it later would become commonplace.