The gram stain

The primary method of distinction among the two main groups of Bacteria remains the Gram stain. Gram-positive and Gram-negative is used in identification classification and taxonomy immediately after morphological study. This important method is based on the ability of the cell wall to retain or lose certain chemicals. The staining procedure was devised in 1884 through Christian Gram. It has since proved to have first a biochemical and then a phylogenetic significance in the Bacteria.

The cells are first fixed to a glass microscope slide and crystal violet is used to stain the preparation. In order to complex the crystal violet with the cell wall, a solution of iodine is then added. If the slide is now washed with alcohol the bound crystal violet stain will be washed out of the thin-walled Gram-negative cells, but cannot pass through the thicker Gram-positive wall. A counter- stain of carbol fuchsin then stains all Gram-negative cells a pale pink while the Gram- positive cells retain their deep violet color.  These color differences can be seen clearly under a light microscope. Most Bacteria react true to their phylogeny with this stain with only a few species such as Paracoccus behaving abnormally. Some attempt has been made to classify the Archaea with the Gram stain, but unfortunately in this kingdom the variability in staining begins at the subgenus level.