Escherichia coli is a classic Gram-negative bacterium. Gram-negative bacteria have a thin peptidoglycan layer sandwiched between an inner and outer membrane, with lipopolysaccharide (LPS/endotoxin) on the outer membrane surface. They stain red/pink in Gram staining (decolourised by acetone-alcohol). Lactobacillus, Staphylococcus aureus, and Bacillus subtilis are all Gram-positive bacteria with thick peptidoglycan walls that retain the crystal violet stain (purple).