Assertion : Gram-negative bacteria do not retain the stain when washed with alcohol. Reason : The outer face of the outer membrane of Gram-negative bacteria contains lipopolysaccharides, a part of which is integrated into the membrane lipids.

The technique of staining bacteria was given by a Dutch microbiologist Christian Gram (1884) and this technique is known as Gram staining. In the cell wall of gram +ve bacteria both horizontal and vertical peptide linkages are present due to which mesh is dense and stain does not come out. Futher outer layer of cell wall of Gram +ve bacteria is made up of techoic acid. In the cell wall of gram -ve bacteria, either horizontal or vertical peptide linkages are present, due to which mesh is loose and hence stain comes out when washed with alcohol. Further outermost layer of cell wall of Gram -ve bacteria is made up lipopolysaccharides.