Gram staining is commonly used in histology to classify bacteria. The process enables to separate most of the bacteria in two groups according to the proportion of membrane peptidoglycans:
–          Gram + bacteria which are rich in peptidoglycans and poor in lipids
–          Gram – bacteria, rich in lipids and poor in peptidoglycans

The first step in the process is effected with crystal violet staining which has an affinity to form an iodine-crystal violet complex from the iodine mordant. Then, an alcoholic solution will weaken and permeabilize the membranes containing most of lipids, in order to clean iodin-Crystal violet complex from Gram – bacteria. It is the step of discoloration. Finally, Gram + bacteria will stained in violet while Gram – bacteria will take the dye of the counterstaining.


Gram staining is commonly used in microbiology to differentiate Gram + and Gram – bacteria. It is an important step in the classification and differentiation of microorganism populations. Gram staining can be used in biopsies to detect infections in the organism, this method is quicker than a classical culture. It is possible to detect:
–          Staphylococci and streptococci (Gram + bacteria) in violet
–          E-Coli (Gram – bacteria) in pink or red according to the counterstaining (Fuchsine or Safranin)

Concerned organs/tissues

Gram staining can be used in all kinds of tissues and organs.