The peptidoglycan layer in Gram-positive bacteria is:
Gram-positive bacteria lack outer membrane but retain an unusually thick peptidoglycan sacculus measuring 20 to 80 nanometers, composed of many layers of glycan strands made of alternating N-acetylglucosamine and N-acetylmuramic acid crosslinked by tetrapeptide side chains containing L-alanine, D-glutamate, meso-diaminopimelic acid or L-lysine, and D-alanine, often further cross-bridged by interpeptide bridges. Embedded within and covalently attached to this mesh are anionic wall teichoic acids, polymers of ribitol or glycerol phosphate linked via phosphodiester bonds, and lipoteichoic acids anchored to membrane diacylglycerol via glycolipid. These polyanionic polymers bind magnesium ions, concentrate cations at membrane surface, modulate activity of peptidoglycan hydrolases and autolysins, regulate division site placement, and serve as antigenic determinants and bacteriophage receptors. Their high negative charge contributes to ion exchange and electrophoretic properties. This architecture explains strong retention of crystal violet-iodine complex during Gram staining, sensitivity to lysozyme that cleaves beta-1,4 bonds, and vulnerability to beta-lactam antibiotics that inhibit transpeptidation step of wall assembly.
Ref: Silhavy et al., The Bacterial Cell Envelope; Vollmer et al., FEMS Microbiol Rev 2008, Peptidoglycan and Teichoic Acids.