Gram staining bacteria technique was developed by Hans Christian Gram (a Danish Bacteriologist) in $1884.$ On the basis of technique, bacteria can be classified into
(a) Gram positive; show positive test with Gram stain.
(b) Gram negative; show negative test with Gram stain.
The step involve in gram staining are summarised as
The composition of bacterial cell wall is basis for Gram staining technique. The cell wall of Gram $( - ve)$ bacteria contain alcohol soluble lipid (lipopolysaccharides) while the cell wall of $(+ ve)$ Gram positive bacteria lack these lipid there retain crystal violet iodine complex used.