A cross-linked complex of polysaccharides and peptides found in the cell walls of bacteria. Gram-positive bacteria (so-called because they color violet when treated appropriately with Gram stain) have a thick layer of a peptidoglycan (or murein), the form of which determines the organism's shape – bacilli (rod shaped), cocci (spherical shaped), or spirilla (helical shaped). In contrast, gram-negative organisms have only a very thin layer of peptidoglycan immediately outside their cell membrane (about one twentieth of the thickness of that found in gram-positive organisms). Surrounding this thin wall of peptidoglycan, however, gram-negative organisms have a bilayered membrane composed of phospholipid and bacterial lipopolysaccharide which has the ability to protect the internal structures of the microbe from damaging chemicals.
