Peptidoglycan, also called murein, is a structural polymer forming the load-bearing framework of the cell wall in most bacteria. It consists of long sugar chains connected by short peptides, producing a continuous network around the cytoplasmic membrane. This network, called the sacculus, helps maintain cellular shape and withstand internal osmotic pressure. Rather than being a static shell, peptidoglycan is assembled, modified, and locally broken down as bacteria grow and divide. Its synthesis is an important target of antibacterial drugs. (nature.com)
Chemical structure
The glycan backbone contains alternating residues of N-acetylglucosamine (GlcNAc or NAG) and N-acetylmuramic acid (MurNAc or NAM), connected by β-(1→4) glycosidic bonds. MurNAc carries a short stem peptide through its lactyl group. Connections between these stems unite neighboring glycan strands into a mechanically coherent structure. Thus, the material combines a carbohydrate backbone with peptide cross-links rather than consisting solely of sugar or protein. (rudnerlab.med.harvard.edu)
A common precursor stem contains five amino acids: L-alanine, D-glutamate, meso-diaminopimelic acid or L-lysine, and two D-alanines. The third residue and any intervening peptide bridge vary among bacteria. In conventional D,D-transpeptidation, the terminal D-alanine is released as a cross-link forms involving the fourth residue of a donor stem. Mature peptidoglycan therefore contains stems of different lengths, and not every stem is cross-linked. D-configured residues distinguish these peptides from the predominantly L-amino-acid chains of ribosomally synthesized proteins. (pubmed.ncbi.nlm.nih.gov)
Although simplified diagrams depict a regular lattice, actual sacculi are heterogeneous. Glycan length, cross-linking density, chemical modifications, and strand orientation differ between species and between regions of an individual cell wall. (nature.com)
Organization in bacterial envelopes
In Gram-positive bacteria, peptidoglycan generally forms a thick wall outside the cytoplasmic membrane. In Gram-negative bacteria, it forms a much thinner layer beneath an outer membrane, within the periplasm. These arrangements place the polymer in different chemical and mechanical environments. Peptidoglycan thickness alone does not describe the complete bacterial envelope: membranes, wall-associated polymers, and surface proteins also contribute to its properties. (nature.com)
The sacculus constrains expansion driven by osmosis and internal turgor. It is nevertheless porous, not a sealed permeability barrier equivalent to a membrane. Atomic-force microscopy of Staphylococcus aureus and Bacillus subtilis has revealed a denser inner surface and a more porous mature exterior. In Escherichia coli, imaging shows that glycan organization is related to cellular morphology without requiring a perfectly crystalline arrangement. (nature.com)
Peptidoglycan is widespread but not universal among bacteria. Mycoplasmas, for example, lack it and lack the principal peptidoglycan-polymerase systems identified in many other bacterial groups. (rudnerlab.med.harvard.edu)
Biosynthesis and growth
Biosynthesis links intracellular precursor production to extracellular polymer assembly. Sugar–peptide precursors are first prepared in the cytoplasm and assembled onto a membrane-associated lipid carrier. The resulting lipid II contains a GlcNAc–MurNAc disaccharide, its stem peptide, and an undecaprenyl lipid attachment. Because its large, hydrophilic headgroup cannot readily cross the membrane unaided, a transporter moves it to the opposite leaflet. In E. coli, MurJ performs this lipid II “flipping” step. (pubmed.ncbi.nlm.nih.gov)
On the external face of the cytoplasmic membrane, enzymes polymerize the glycan backbone and cross-link the peptides. Two major synthetic systems are recognized. Class A penicillin-binding proteins combine glycan-polymerizing and peptide-cross-linking activities. SEDS-family polymerases, including RodA, operate with class B penicillin-binding proteins, which provide transpeptidase activity. The identification of RodA as a polymerase established that wall synthesis cannot be attributed exclusively to class A proteins. (nature.com)
The location of synthesis helps determine cell shape. Elongation machinery adds wall material during growth of many rod-shaped bacteria, whereas division machinery constructs the septum between daughter cells. Local recruitment and regulation of synthetic components coordinate peptidoglycan deposition with cell division. (nature.com)
Remodeling and susceptibility to antibiotics
Expansion requires controlled cleavage of existing bonds so that new material can enter the network. Hydrolytic enzymes modify glycan chains and peptide connections, allowing changes in wall architecture while preserving overall integrity. Peptidoglycan is therefore a dynamic material whose structure reflects both synthesis and degradation. (nature.com)
Several antibiotics interfere with its construction. β-lactams, including penicillin, inhibit susceptible transpeptidases and thereby disrupt cross-link formation. Vancomycin instead recognizes the D-Ala–D-Ala terminus of precursors. Replacement of this terminus with D-Ala–D-lactate markedly reduces vancomycin binding, illustrating one molecular mechanism of antibiotic resistance. These mechanisms affect wall assembly rather than simply dissolving an existing sacculus. (pubmed.ncbi.nlm.nih.gov)
Immune recognition and investigation
Lysozyme attacks the glycan backbone through hydrolysis. Bacterial modifications can reduce susceptibility: experiments in S. aureus show that MurNAc O-acetylation, wall teichoic acids, and extensive cross-linking contribute to resistance to lysozyme. (pmc.ncbi.nlm.nih.gov)
Peptidoglycan fragments also provide signals to innate immunity. Intracellular NOD1 responds to diaminopimelic-acid-containing peptide motifs, while NOD2 detects muramyl dipeptide, a motif distributed across Gram-positive and Gram-negative peptidoglycans. Recognition depends on particular molecular fragments rather than merely on the presence of an intact wall. (nature.com)
Experimental analysis combines imaging with chemical measurements. Fluorescent D-amino acids label wall material in living bacteria, revealing spatial patterns of incorporation. Chromatography and mass spectrometry characterize the resulting muropeptides, while atomic-force microscopy examines the organization of purified sacculi and living cell surfaces. (pubmed.ncbi.nlm.nih.gov)