Structure of Peptidoglycan (Murein)
The foundation of strength for any bacterium is peptidoglycan (synonyms: murein, mucopeptide). It forms a rigid fibrous network around the cytoplasmic membrane.
This molecular structure consists of two main parts:
- Glycan backbone. Composed of parallel chains of repeating disaccharides: N-acetylglucosamine (NAG) and the unique N-acetylmuramic acid (NAM). They are connected by a glycosidic bond. This exact bond can be cleaved by the enzyme acetylmuramidase (lysozyme).
- Peptide cross-bridge. Side tails—tetrapeptides (chains of four amino acids)—attach to N-acetylmuramic acid molecules. Important detail: these tails alternate between L- and D-isomers of amino acids.
A typical tetrapeptide sequence (using Gram-negative E. coli as an example) is: L-alanine, D-glutamic acid, meso-diaminopimelic acid (mDAP), and D-alanine. Adjacent chains cross-link with each other to form a monolithic framework.
Gram-Positive Bacteria (Gram+)
The cell envelope of Gram-positive microorganisms (e.g., genus Staphylococcus) is massive and tightly apposed to the membrane. When stained using the method introduced by Hans Christian Gram in 1884, they appear blue-purple.
Key features:
- Multilayered peptidoglycan. It accounts for 40–90% of the dry weight of the cell wall. Minor components—proteins, lipids, and polysaccharides—are present in small amounts.
- Complex cross-linking. Tetrapeptides are connected not directly, but via a polypeptide interpeptide bridge (e.g., a pentaglycine bridge of 5 glycine residues), making the structure very rigid.
- Amino acid difference. Instead of meso-diaminopimelic acid, lysine is frequently found in the tetrapeptides.
- Teichoic acids. The envelope is permeated by polymers of glycerol phosphate and ribitol phosphate. Teichoic acids are covalently linked to murein, while lipoteichoic acids are hydrophobically anchored directly into the membrane. They create a strong negative charge, assist in cell division, regulate autolysis, and mediate adhesion to host tissues.
Gram-Negative Bacteria (Gram-)
These microorganisms (e.g., E. coli) stain red in the Gram stain. Their protective apparatus is more complex, although the peptidoglycan layer itself is significantly thinner.
Key features:
- Thin framework. Peptidoglycan is located in the periplasmic space (periplasm).
- Direct cross-linking. D-alanine of one glycan chain connects directly to the meso-diaminopimelic acid of an adjacent chain, without additional peptide bridges.
- Outer membrane. This is a critical barrier located superficial to the wall and linked to it via lipoproteins. It contains porins (transport channels) and lipopolysaccharide (LPS).
- LPS structure. Lipopolysaccharide includes an O-specific chain (O antigen), a core polysaccharide, and Lipid A.
Limitations of the Gram Stain and Visualization
Despite the ubiquitous use of the Gram stain, the staining color does not always strictly correspond to the actual structural type. Exceptions exist: for example, bacteria of the genus Mobiluncus and certain spore-forming species have a typical Gram-positive structure yet stain as Gram-negative.
Therefore, for microbial taxonomy, the chemical composition and molecular organization of the cell envelope are much more reliable criteria than laboratory staining results.
The true picture can be seen using electron microscopy. Ultrathin sections clearly visualize differences in envelope thickness between Gram+ and Gram- bacteria, and the nucleoid (genetic material) can be discerned in the center of the cytoplasm.