The main purpose of the peptidoglycan sacculus is to maintain bacterial shape and to counteract the internal pressure of the bacterial cell, which is approximately 3–5 atm in Gram-negative bacteria and up to 25 atm in Gram-positive bacteria. This is reflected by the thickness of the peptidoglycan sacculus. Experimental evidence suggests that the sacculus is mainly single layered in Gram-negatives while Gram-positives have up to 40 layers of peptidoglycan.

Peptidoglycan also serves as an anchor for proteins. In Gram-negatives the only protein known to be covalently attached to the peptidoglycan is Braun's lipoprotein (Lpp), which links the sacculus to the outer membrane. Approximately one-third of the Lpp is covalently bound to the alpha-carboxyl-group of meso-diaminopimelic acid (m-A2pm) of the stem peptide by the episilon-amino group of the Lys at the Lpp C-terminus. The other two-thirds are freely associated with the outer membrane. Covalent binding is achieved by an l,d-transpeptidase reaction catalysed by three different proteins: ErfK, YcfS and YbiS, of which the latter seems to convey the main transpeptidase activity. During stationary growth the abundance of the bound form increases. In Gram-positive bacteria, proteins, capsular polysaccharides, and teichoic acids are covalently and non-covalently associated with peptidoglycan. These molecules are responsible for bacteria–host interactions and virulence. The covalent attachment of proteins is mediated by sortases, which recognize a specific cell wall sorting signal (CWS) located in the C-terminus of the attached protein.

S. aureus contains two different sortases: SrtA, recognizing the CWS 'LPXTG', anchors at least 21 proteins to peptidoglycan including protein A (Spa), fibronectin-binding proteins (Fnbp) A and B, clumping factor (Clf) A and B, and collagen adhesion protein (Cna), all of which are responsible for the manifestation of infections. SrtA directly anchors the proteins to the murein precursor molecule lipid II in a two-step transacetylation reaction, thus forming an amide bond between threonine (Thr) of the LPXTG-motif and glycine (Gly) at position five of the pentaglycine-bridge. In many Gram-positive bacteria this pathway is universal.

The second sortase of S. aureus is SrtB, whose only substrate is the NPQTN-containing protein IsdC. This iron-uptake protein is attached to non-cross-linked Gly5 of mature peptidoglycan by an amide bond between Thr and Gly. In other Gram-positive bacteria sortases of the C-family polymerize fimbriae and pili and anchor them to the murein sacculus. Sortases of the d-family play a role in developmental processes, e.g. during sporulation of Bacillus anthracis and mycelium formation in Streptomyces coelicolor. The covalent amide bond is always formed between the Thr and the Gly5.

from Ute Bertsche in Bacterial Polysaccharides

Further reading: Bacterial Polysaccharides: Current Innovations and Future Trends

Labels: peptidoglycan