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Peptidoglycan is also known as murein. They are polymers and are composed of sugars and proteins which form a web-like structure outside of the cell membrane of the bacterial except the archaea. Structurally, peptidoglycan is composed of alternating units of β (1–4) linked N-acetylglucosamine and N-acetylmuramic acid. The N-acetylmuramic acid is covalently linked to small polypeptides that consist of alanine and other unusual amino acids such as, D-amino acid and L-diaminopimelic acid to form a 3D network. Interestingly, these unusual amino acids are generally not found in other proteins, thus provide high protection from peptidase enzymes. The 3D network of the peptidoglycan provides elasticity, porosity and scaffolding to the bacterial cell wall. Other bacterial polysaccharides such as capsular polysaccharides and teichoic acid have been anchored in peptidoglycan, thus giving a covering layer of the glycan to the bacteria which directly exposed to their environment. So, peptidoglycan is most prominent in Gram positive bacteria with 99% dry molecular weight and 10% in gram negative bacteria [55, 56].

Figure 2.4 The given illustration depicts the bacterial cell wall that composed of peptidoglycan and LPS. Peptidoglycan is made of alternating units of β (1–4) linked N-acetylglucosamine and N-acetylmuramic acid. LPS has further three components such as, Core polysaccharides, O-antigen, and lipid-A which is notorious for their pathogenicity [66].