The most prominent bacterial structural characteristic is the cell wall. Bacteria can be divided into two groups (gram-positive and gram-negative) based on differences in cell wall structure as revealed by Gram staining. In a Gram stain, a series of dyes are applied to stain cell walls. Gram-negative bacteria have an outer, lipopolysaccharide-containing membrane and stain pink. They also have a thin peptidoglycan layer located in the periplasm (the region between the outer and cytoplasmic membranes). Gram-positive bacteria lack this covering, but possess a cell wall containing a thick peptidoglycan (called Murein in older sources) layer and teichoic acids; they stain purple. Gram-positive bacteria are more susceptible to antibiotics, while gram-negative bacteria are difficult to treat with antibiotics.
