Other spiral-shaped bacteria include the spirochaetes , such as Treponema pallidum which causes syphilis. However spirochaetes have a different type of motility from that of the common spiral bacteria. The Gram reaction is named after the Danish physician, Christian Gram, who developed this staining technique in 1884. It involves a series of simple steps. Bacterial cells are dried onto a glass slide and stained with crystal violet, then washed briefly in water.
Iodine solution is added so that the iodine forms a complex with crystal violet in the cells.
Alcohol or acetone is added to solubilise the crystal violet - iodine complex. The cells are counterstained with safranin, then rinsed and dried for microscopy. The method separates bacteria into two types. The thickness of this wall blocks the escape of the crystal violet-iodine complex when the cells are washed with alcohol or acetone. The crystal violet-iodine complex is easily lost through the LPS and thin peptidoglycan layer when the cells are treated with a solvent.