Gram (+)ve and Gram (-)ve bacteria separate on the basis of their cell wall composition. Christian Gram on the basis of staining behaviour of the cell wall with Gram’s stain, grouped bacteria into Gram (+)ve and Gram (–)ve type. The bacteria which retain blue or purple colour after staining are Gram (+)ve e.g. Bacillus subtilis and which loses blue colour is Gram ( – )ve and retain counter stain saffranine appear red eg. E.coli. In Gram (–)ve, the stain is washed due to high lipid content in the cell wall having a thick wall, Gram ( + )ve has a single-layered cell wall.

Gram stain → Crystal Violet