Gram staining is a method for staining and identifying bacteria. It is based on the differential retention of a crystal violet-iodine complex within the cell membrane. The cell wall of gram-positive cells have a thick peptidoglycan layer that retains crystal violet after an alcohol wash (staining black-blue or purple by a counterstain). The cell wall of gram-negative cells have a thinner peptidoglycan layer that does not retain crystal violet after an alcohol wash (staining red after counterstain). Some cells have variable staining in culture (e.g. Bacillus, Clostridium) while others have indeterminate staining due to unique aspects of their cell membrane or wall (e.g. Mycobacterium, Mycoplasma).