Peptidoglycan, also known as murein, is a homogenous layer lying outside the plasma membrane in prokaryotes. The peptidoglycan layer is thicker in Gram-positive bacteria (20 to 80 nm) than in Gram-negative bacteria (7 to 8 nm). It forms around 90% and 10% of dry weight of gram positive and gram negative bacteria respectively.

The peptidoglycan layer in the bacterial cell wall is a lattice structure formed from linear chains of two alternating amino sugars, namely N-acetyl glucosamine (GlcNAc) and N-acetyl muramic acid (MurNAc). Each MurNAc is attached to a short (4 to 5 residues) amino acid chain. Cross links between amino acids in different linear amino sugar chains by an enzyme known as transpeptidase result in a 2-dimensional sheet that is strong and rigid. The exact amino acid sequence and the exact overall structure vary with the bacterial species.

Peptidoglycan serves a structural role in bacterial cell walls giving bacteria shape, strength and counteracting the osmotic pressure of the cytoplasm. It is also involved in binary fission of the bacterial cell. The formation of the peptidoglycan layer in bacteria, specifically the crosslinking enzyme transpeptidase, is the target for drugs such as penicillin.