Peptidoglycan

Peptidoglycan, also called murein, is the large polymer that makes up the cell wall of nearly all bacteria. It forms one continuous, bag-shaped molecule, the sacculus, that encloses the cell just outside the plasma membrane. Its backbone is made of long sugar chains in which two modified sugars alternate: NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid). A short chain of amino acids hangs from each NAM, and these peptides are cross-linked to peptides on neighboring chains, tying the strands into a tough net.

This net keeps the cell in shape and stops it from bursting as water flows in by osmosis. Gram-positive bacteria have a thick wall of many peptidoglycan layers, whereas Gram-negative bacteria have only a thin layer beneath an outer membrane, and this difference is what the Gram stain reveals. Archaea do not make peptidoglycan, though some have a look-alike called pseudopeptidoglycan.

Because human cells contain no peptidoglycan, it makes an ideal drug target. Penicillins and other beta-lactam antibiotics block the enzymes that form the cross-links, and vancomycin binds the building blocks, so the weakened wall gives way. The enzyme lysozyme, found in tears and mucus, attacks the sugar backbone.