The peptidoglycan layer of the Gram-negative cell envelope. In Gram-negative cells the peptidoglycan is relatively thin (1-2nm) and is linked to the outer membrane by lipoproteins. In Gram-negative cells the peptidoglycan is too thin to retain the primary stain in the Gram staining procedure and therefore cells appear red after Gram stain. [ https://www.worldcat.org/search?q=bn%3A0815108893 ]