Peptidoglycan

Peptidoglycan is the essential cell wall material found mostly in Gram-positive bacteria. It is a linear glycan (carbohydrate) backbone cross-linked by peptide stems. This material creates a mesh or sacculus surrounding the cytoplasmic membrane where it is composed of teichoic acids, polysaccharides and proteins. 

The peptidoglycan strands are constructed by bonding alternate units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) residues via beta 1-4 linkages. The peptide chains are linked covalently through their N-terminus to the lactyl group of MurNAc (Chapot-Chartier & Kulakauskas, 2014).

Its role is to maintain cell shape and rigidity, turgor pressure and integrity especially during growth and division. The cell walls of Gram-positive bacteria may contain up to 40 layers of peptidoglycan, conferring significant mechanical strength. It is also part of the interface between a bacterium and its environment (Stewart-Tull, 1980).

The peptidoglycan part is also implicated in the performance of probiotics and postbiotics. Lactic acid bacteria (LAB) are especially important in this role.

The macromolecular peptidoglycan is assembled from monomeric units synthesized in the cytoplasm. This is an N-acetylglucosamine and N-acetylmuramic acid disaccharide with a five amino acid stem. When the monomer is flipped into the periplasm, it is added to the glycan chain by transglycosylation and a portion of the peptide stems are linked together by transpeptidation.

The Amino-Acids of Peptidoglycan

The amino acids making up the peptide stem varies between species. They are generally attached to muramic acid in the order L-alanine, D-glutamic acid, meso-diaminopimelic acid, D-alanine, D-alanine, with L-lysine taking the place of diaminopimelic acid in some Gram-positives. Cross-linking occurs through the free amine of the third amino acid linking either the third or fourth amino acid directly or through linker amino acids. Other modifications include amidation of amino acids, o-acetylation and/or N-deacetylation of sugars.

Analysis

Peptidoglycan is extracted from lactobacilli using mechanical and enzymic extraction. The cell wall is stripped of its lipids using a series of solvent extractions involving methanol-chloroform (1:1). The crude preparation is then treated with proteases and nucleases to remove wall bound proteins and nucleic acids. The amino-acid composition is established via SDS-PAGE (Wang et al., 2018).

Function

Peptidoglycan is part of a group of structures recognised primarily by the immune system. These are essentially part of the structures found in the cell membrane. These include as well as peptidoglycan, other surface molecules, such as lipopolysaccharide (LPS) and peptidoglycan recognition protein (PGRP), that are known to elicit immune reactions ranging from cytokine release to fever.

Peptidoglycans activate the Toll-like receptor 2 (TLR2), that is present in mammalian cells, and they can be used for the stimulation of lymphocytes. Peptidoglycans also function as antagonists of Poly(I:C). Peptidoglycans may be used to estimate the activity of lysing enzymes such as lyticase.

Antibiotics such as penicillin with its beta-lactam structure interfere with cell wall synthesis by weakening the peptidoglycan scaffold within the bacterial wall and the structural integ  .  

References

Chapot-Chartier, M.P., Kulakauskas, S. (2014) Cell wall structure and function in lactic acid bacteria. Microb. Cell Fact. 13:S9. (Article) 

Stewart-Tull DES. 1980. The Immunological Activities of Bacterial Peptidoglycans. Annu. Rev. Microbiol.. 34(1):311- 340 (Article).  

Wang S, Han X, Zhang L, Zhang Y, Li H, Jiao Y. (2018) Whole peptidoglycan extracts from the Lactobacillus paracasei  subsp. paracasei M5 strain exert anticancer activity in vitro. Biomed Res Int. 2018; pp. 1–12.   .