peptidoglycan

Peptidoglycan (murein) consists of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugar residues linked by beta-1,4 glycosidic bonds, with short peptide chains cross-linked via transpeptidation (catalyzed by penicillin-binding proteins, PBPs). Gram-positive bacteria have a thick peptidoglycan layer (multiple layers, often with teichoic acids embedded) that retains crystal violet stain, while gram-negative bacteria have a thin peptidoglycan layer sandwiched between inner and outer membranes, contributing to their resistance to certain antibiotics and different staining properties. Peptidoglycan synthesis occurs in three stages: (1) cytoplasmic synthesis of NAM-pentapeptide precursor, (2) transport across membrane via bactoprenol carrier, (3) polymerization and cross-linking by transglycosylases and transpeptidases (PBPs) in the periplasmic space/cell wall. Clinically, peptidoglycan synthesis is the target of major antibiotic classes: beta-lactams (penicillins, cephalosporins, carbapenems) inhibit transpeptidase (PBP) cross-linking; vancomycin binds D-Ala-D-Ala terminus of the pentapeptide, blocking transpeptidation and transglycosylation; bacitracin inhibits dephosphorylation of the bactoprenol carrier. Peptidoglycan fragments (muramyl dipeptides) are recognized by host innate immune receptors (NOD1/NOD2) triggering inflammatory responses, and can activate complement and stimulate fever (endogenous pyrogen release from macrophages). Lysozyme in tears, saliva, and secretions cleaves the beta-1,4 bonds, providing innate antibacterial defense—bacteria lacking peptidoglycan (e.g., Mycoplasma) are inherently resistant to beta-lactams and lysozyme. Mycobacteria have peptidoglycan plus a unique mycolic acid layer, altering antibiotic susceptibility. Understanding peptidoglycan structure is essential for explaining mechanisms of antibiotic action, bacterial resistance mechanisms (e.g., altered PBPs in MRSA, altered D-Ala-D-Lac in VRE), and gram stain differences.