Gram-negative bacterial cell wall, illustration. The horizontal layers include both an external and an internal membrane (blue), both containing transmembrane proteins (green, yellow and purple). The membranes are separated by a thin peptidoglycan layer (purple rods). The outer surface of the external membrane is often a lipopolysaccharide layer with lipids (purple) in the membrane, and long saccharide side chains (red) extending out. This is termed a Gram-negative cell wall because it does not retain the Gram stain that helps identify microbial life.