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Peptidoglycan is a polymer composed of alternating N-acetylglucosamine and N-acetylmuramic acid units linked by amino acids, forming a flexible mesh. This structure provides mechanical strength, maintains cell shape, and protects bacteria from osmotic pressure. Peptidoglycan synthesis is essential for cell wall formation and bacterial survival.

Gram-positive bacteria have thick peptidoglycan layers with embedded teichoic acids that facilitate ion uptake and defense. Gram-negative bacteria possess thinner peptidoglycan layers surrounded by an outer membrane containing lipopolysaccharides. These structural differences cause Gram-positive bacteria to retain crystal violet dye (appearing purple), while Gram-negative bacteria retain the counterstain (appearing pink).

Teichoic acids are embedded within the thick peptidoglycan layers of Gram-positive bacteria, where they facilitate cation transport and protect cells from harmful substances, including antibiotics and host defense molecules. These molecules enhance the structural integrity and defensive capabilities of the cell wall.

Penicillin targets peptidoglycan synthesis, and Gram-positive bacteria have thick, exposed peptidoglycan layers that are readily accessible to the antibiotic. Gram-negative bacteria have a thinner peptidoglycan layer protected by an outer membrane, which acts as a barrier that prevents penicillin from reaching its target, conferring intrinsic resistance.

Lipopolysaccharides (LPS) are components of the outer membrane in Gram-negative bacteria, providing structural integrity and acting as a selective barrier. LPS also possesses endotoxin activity that triggers the host's immune response during infection, making it a significant virulence factor in Gram-negative pathogens.

The bacterial cell wall is a rigid structure surrounding the plasma membrane that maintains the cell's shape and provides mechanical strength through its peptidoglycan mesh. This rigid framework prevents cell lysis by resisting osmotic pressure, allowing bacteria to survive in hypotonic environments where water would otherwise enter and rupture the cell.

The Gram stain exploits differences in cell wall structure to differentiate bacteria. Gram-positive bacteria retain the crystal violet dye due to their thick peptidoglycan layer, appearing purple. Gram-negative bacteria have a thinner peptidoglycan layer and outer membrane that allow the dye to wash out, so they retain the counterstain and appear pink.