Gram Staining
Christian Gram invented a staining system for visualisation of bacteria using light microscopy  based on the differential retention of a crystal violet-iodine complex within the cell membrane. Gram positive bacteria retain the stain when washed with alcohol or acetone whereas Gram negative bacteria lose the stain as a result of the washing process. Gram negative bacteria are however able to be visualised with a pink counter stain called safranin.  The Gram staining characteristics of different bacterial species is still used by microbiologists but  the information obtained is merely descriptive; it can not be used to identify cell wall structure or composition nor can it be used to classify organisms by their metabolic characteristics.  For example some methanogens are gram positive and others are gram negative.