# Peptidoglycan
**Peptidoglycan** (also called murein) is a polymer of glycan strands cross-linked by short peptides that forms a single, large macromolecular sacculus around the cytoplasmic membrane of most bacteria. Its major function is to resist the intracellular osmotic pressure (turgor) generated by the cytosolic contents; it also defines bacterial cell shape and provides protection from environmental threats.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) Because it is essential, unique to bacteria, and accessible outside the cytoplasm, peptidoglycan biosynthesis is a validated target for antibiotics, including the β-lactams and the glycopeptides.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4632607/) • [3](https://www.jbc.org/article/S0021925817495317/pdf)
| Key fact | Detail |
|---|---|
| Chemical makeup | Linear glycan strands of alternating N-acetylmuramic acid (MurNAc) and N-acetylglucosamine (GlcNAc) joined by β-1,4 glycosidic bonds, cross-linked by short peptide stems usually 2 to 5 residues long[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) |
| E. coli stem peptide | L-alanine, γ-D-glutamate, meso-diaminopimelic acid (mDAP), and D-alanine–D-alanine, with an isopeptide linkage between D-glu and mDAP[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) |
| Gram-positive wall | Many cross-linked layers, 20 to 100 nm thick, external to the single cell membrane, interwoven with teichoic acids[4](https://journals.asm.org/doi/10.1128/jb.00359-25) |
| Gram-negative wall | A few layers, approximately 1 to 10 nm thick, in the periplasm between inner and outer membranes; predominantly a single layer about 2.5 nm thick in a 15-nm-wide periplasm[4](https://journals.asm.org/doi/10.1128/jb.00359-25) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) |
| Precursor transport | The lipid-linked precursor Lipid II is synthesized in the inner leaflet of the cytoplasmic membrane and translocated across the bilayer by the MurJ flippase[5](https://pubmed.ncbi.nlm.nih.gov/35274942/) |
| Polymerization enzymes | Glycosyltransferase (transglycosylation) reactions polymerize the glycan strands and transpeptidation cross-links the peptides; the SEDS proteins RodA and FtsW are glycosyltransferases for elongation and division[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4632607/) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10216114/) |
## Chemical structure
Peptidoglycan is a polymer of several linear glycan strands covalently cross-linked to each other by short peptide chains. Each glycan strand is a long chain of disaccharide repeats, each containing one MurNAc and one GlcNAc residue linked by β-1,4 glycosidic bonds. The lactoyl moieties of MurNAc carry the peptide stems, which are usually two to five amino acid residues long.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) The basic subunit, the muropeptide, is this disaccharide with its attached peptide; adjacent glycan strands are cross-linked via the peptide side chains to produce a lattice-like sacculus surrounding the entire cell.[4](https://journals.asm.org/doi/10.1128/jb.00359-25)
The chemistry is unusual for a biological polymer: it contains the rare sugar MurNAc, γ-bonded D-glutamate, L–D and even D–D peptide bonds, and nonprotein amino acids such as mDAP.[8](https://academic.oup.com/femsre/article/32/2/149/2683904) Strand length and cross-linking vary with species, strain, and growth conditions. In *Escherichia coli*, glycan chains average 25 to 35 disaccharide units with a broad length distribution, and most cross-links join the mDAP at position 3 of one stem to the D-alanine at position 4 of a neighboring stem.[9](https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00331/full) In *Staphylococcus aureus*, a lysine occupies stem position 3 and a pentaglycine interpeptide bridges the lysine amino group to the D-alanine of a neighboring chain; most *S. aureus* glycan chains are only 3 to 10 disaccharides long, with a maximum of 23 to 26.[9](https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00331/full) Glycan strands also carry secondary modifications such as N-deacetylation, O-acetylation, and N-glycolylation; [Gram-negative bacteria](https://www.edgepedia.org/gram-negative-bacteria) show fewer of these, mostly in pathogenic species where they help evade lysis by host lysozymes.[8](https://academic.oup.com/femsre/article/32/2/149/2683904) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/)
## Architecture in Gram-positive and Gram-negative walls
Bacteria are classified as Gram-positive or Gram-negative largely on differences in peptidoglycan thickness and location. Gram-positive cells have many cross-linked peptidoglycan layers, 20 to 100 nm thick, outside the single cell membrane; the layer is interwoven with teichoic acids and may be capped by capsular polysaccharide or an S-layer of proteins.[4](https://journals.asm.org/doi/10.1128/jb.00359-25) Gram-negative cells hold only a few layers, roughly 1 to 10 nm thick, in the periplasm between the inner and outer membranes; in several Gram-negative bacteria the sacculus is a single layer about 2.5 nm thick.[4](https://journals.asm.org/doi/10.1128/jb.00359-25) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) In the Gram-negative sacculus, the glycans lie parallel to the cell surface, roughly perpendicular to the long axis, encircling the cell in disorganized hoop-like fashion.[10](https://pubmed.ncbi.nlm.nih.gov/19033194/) Electron cryotomography supports a layered model in Gram-negative bacteria, while rod-shaped *Bacillus subtilis* shows a more elaborate architecture with bands of coiled glycan-strand bundles.[11](https://pubmed.ncbi.nlm.nih.gov/20060721/?dopt=Abstract)
## Biosynthesis
De novo synthesis begins in the cytoplasm with the formation of UDP-GlcNAc, a shared precursor for peptidoglycan and, in Gram-negative bacteria, outer-membrane biosynthesis. The subsequent Mur ligases build the stem peptide onto the sugar; their genes sit in the large dcw (division-cell-wall) operon together with genes for cell division, although the ligases themselves are not known to be regulated.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) The final lipid-linked precursor, Lipid II, is assembled in the inner leaflet of the cytoplasmic membrane and then flipped across the bilayer by the MurJ flippase so that polymerization can occur outside.[5](https://pubmed.ncbi.nlm.nih.gov/35274942/)
On the outer face, two enzymatic activities build the wall: glycosyltransferase (transglycosylation) reactions polymerize the glycan strands from Lipid II, and transpeptidation cross-links the peptide stems.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4632607/) Class A penicillin-binding proteins such as PBP1a and PBP1b carry both activities, while the SEDS proteins (shape, elongation, division, and sporulation) RodA and FtsW are dedicated glycosyltransferases that work with class B penicillin-binding proteins; the discovery of the RodA-PBP2 scaffold synthase pair changed the concept of cell wall assembly, suggesting PBP1a and PBP1b fill gaps created by scaffold synthesis.[5](https://pubmed.ncbi.nlm.nih.gov/35274942/) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10216114/) Growth is organized by multiprotein machines: the elongasome, present in rod-shaped bacteria, inserts new material along the long axis, while septal synthesis constricts the cell at division, and synthesis complexes interdepend with cytoskeletal elements such as MreB and FtsZ.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC10216114/) • [12](https://www.nature.com/articles/nrmicro2677)
## Hydrolases and remodeling
The wall is remodeled continuously during growth and division, requiring a balance between degradation and biosynthesis. [Cell wall](https://www.edgepedia.org/cell-wall) hydrolases fall into three classes by the bond they cleave: amidases cut the amide bond between MurNAc and the L-alanine of the stem peptide, glycosidases hydrolyze the glycosidic linkages, and peptidases cut amide bonds between amino acids within the peptide chain.[9](https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00331/full) These enzymes, historically called lysins (endolysins, exolysins, and autolysins), often have modular architectures combining several catalytic domains plus cell-wall binding domains.[9](https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00331/full) The first such hydrolase, lysozyme, was discovered by [Alexander Fleming](https://www.edgepedia.org/alexander-fleming) in 1921; it cleaves the β-(1-4)-glycosidic bond between MurNAc and GlcNAc and serves as part of animal antibacterial defense.[9](https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00331/full)
Peptidoglycan is also recycled: bacteria consume their own wall during turnover, and non-canonical amino acids released into the medium can act as signaling molecules influencing biofilm formation, cellular metabolism, and interaction with the host.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/) Growth of the sacculus is sensitive to mechanical force and nutritional status, and D-amino acids participate in sacculus remodeling.[12](https://www.nature.com/articles/nrmicro2677)
## Target of antibiotics
Resistance to these drug classes, including methicillin-resistant *S. aureus* and vancomycin-resistant enterococci, has driven continued work on the cell wall as an attack surface and on new classes of antibiotics informed by recent discoveries in peptidoglycan biogenesis.[5](https://pubmed.ncbi.nlm.nih.gov/35274942/)
## References
1. [Peptidoglycan: Structure, Synthesis, and Regulation, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11168573/)
2. [Activities and regulation of peptidoglycan synthases, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4632607/)
3. [Bacterial cell wall synthesis and antibiotic targets, JBC Reviews](https://www.jbc.org/article/S0021925817495317/pdf)
4. [Compositional analysis of bacterial peptidoglycan: insights from peptidoglycomics, Journal of Bacteriology](https://journals.asm.org/doi/10.1128/jb.00359-25)
5. [The Bacterial Cell Wall: From Lipid II Flipping to Polymerization, PubMed](https://pubmed.ncbi.nlm.nih.gov/35274942/)
6. [Recent Advances in Peptidoglycan Synthesis and Regulation in Bacteria, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10216114/)
7. [Treatment of Bacterial Infections with β-Lactams: Cooperation with Innate Immunity, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC9933636/)
8. [Peptidoglycan structure and architecture, FEMS Microbiology Reviews](https://academic.oup.com/femsre/article/32/2/149/2683904)
9. [Cell Wall Hydrolases in Bacteria, Frontiers in Microbiology](https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.00331/full)
10. [Molecular organization of Gram-negative peptidoglycan, PubMed](https://pubmed.ncbi.nlm.nih.gov/19033194/)
11. [Architecture of peptidoglycan: more data and more models, PubMed](https://pubmed.ncbi.nlm.nih.gov/20060721/?dopt=Abstract)
12. [From the regulation of peptidoglycan synthesis to bacterial growth and morphology, Nature Reviews Microbiology](https://www.nature.com/articles/nrmicro2677)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Bacteria › Bacterial cell biology and structure*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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