Bacterial Cell Wall

Differential staining separates bacteria into two groups based on how their cell walls respond to the crystal violet–iodine complex. Gram-positive bacteria possess a thick, multilayered peptidoglycan (murein) wall that retains the dye–iodine complex even after alcohol washing, so they appear purple, a principle reflected in NCERT Class 11, Chapter 8 (Cell: The Unit of Life) discussion of the bacterial cell wall. The option about an outer lipopolysaccharide membrane describes Gram-negative bacteria, which have a thin peptidoglycan layer and lose the stain on decolourisation, appearing pink after counterstaining. The choice claiming a complete absence of a cell wall is wrong because almost all true bacteria except mycoplasmas possess walls, and wall-less cells would not give a Gram reaction. Comparing bacterial walls to plant cellulose walls is incorrect, since bacterial walls are built from peptidoglycan, not cellulose. The Gram-negative envelope additionally carries an outer membrane of lipopolysaccharide outside its thin wall, and the alcohol used in decolourisation dissolves part of that lipid layer, allowing the dye to escape so the cell takes up the pink counterstain. This structural difference is also why many antibiotics that target peptidoglycan synthesis act more strongly on Gram-positive forms. A logical check confirms the answer: dye retention depends on wall thickness, and only a thick peptidoglycan meshwork can physically trap the large crystal violet–iodine complex.