Structure of peptidoglycan (murein)
The structural strength of any bacterium is based on peptidoglycan (synonyms: murein, mucopeptide). It forms a rigid fibrous network around the cytoplasmic membrane.
This molecular structure consists of two main parts:
- Glycan backbone. It consists of parallel chains of repeating disaccharides: N-acetylglucosamine and the unique N-acetylmuramic acid. They are linked by a glycosidic bond. This very bond is cleaved by the enzyme acetylmuramidase (lysozyme).
- Peptide cross-link. Side chains—tetrapeptides (chains of four amino acids)—are attached to N-acetylmuramic acid molecules. An important detail: these chains alternate between L- and D-isomers of amino acids.
A typical tetrapeptide sequence (using the Gram-negative E. coli as an example) is as follows: L-alanine, D-glutamic acid, meso-diaminopimelic acid (mDAP), and D-alanine. Neighboring chains are cross-linked to form a monolithic framework.
Gram-positive bacteria (G+)
The cell wall of Gram-positive microorganisms (e.g., genus Staphylococcus) is massive and tightly adheres to the membrane. When stained using the method proposed by H. Gram in 1884, they turn blue-purple.
Key features:
- Multilayered peptidoglycan. It accounts for 40–90% of the dry mass of the 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 extremely rigid.
- Amino acid difference. Lysine is frequently found in tetrapeptides instead of meso-diaminopimelic acid.
- Teichoic acids. The cell wall is permeated with polymers of glycerol phosphate and ribitol phosphate. Teichoic acids are covalently linked to murein, while lipoteichoic acids are hydrophobically anchored directly in the membrane. They create a strong negative charge, assist in cell division, regulate autolysis, and ensure adhesion to host tissues.
Gram-negative bacteria (G-)
These microorganisms (e.g., E. coli) stain red with the Gram stain. Their protective apparatus has a more complex organization, although the peptidoglycan layer itself is significantly thinner.
Key features:
- Thin framework. Peptidoglycan is located in the periplasmic space (periplasm).
- Direct cross-link. D-alanine of one glycan chain is directly linked to the meso-diaminopimelic acid of the adjacent chain, without additional peptide bridges.
- Outer membrane. This is a crucial barrier located superficial to the cell wall and linked to it via lipoproteins. It contains porins (transport channels) and lipopolysaccharide (LPS).
- LPS structure. Lipopolysaccharide comprises the O-specific chain (O-antigen), core polysaccharide, and Lipid A.
Gram stain limitations and visualization
Despite the widespread use of the Gram stain, the staining color does not always strictly correspond to the actual type of wall structure. There are exceptions: for example, bacteria of the genus Mobiluncus and certain spore-forming species have a typical Gram-positive structure yet stain Gram-negative.
Therefore, for microbial taxonomy, the chemical composition and molecular organization of the cell wall serve as a much more reliable criterion than laboratory staining results.
The true picture can be visualized using electron microscopy. Ultrathin sections clearly demonstrate differences in wall thickness between G+ and G- bacteria, while the nucleoid (genetic material) can be discerned in the center of the cytoplasm.
Briefly: answers to topical questions
What are the functions of the outer membrane in Gram-negative bacteria?
- Barrier function (barriere) — restriction of the penetration of macromolecular compounds and drugs due to the outer membrane with small-diameter pores.
- Transport function (transport) — transport of substances through porin channels (porins).
- Structural connection (structura) — linkage of the outer membrane to peptidoglycan via lipoproteins.
- Toxigenicity (toxicitas) — presence of lipopolysaccharide (LPS), which includes the endotoxin Lipid A, O-specific chain, and core.
Which antibiotic groups block peptidoglycan synthesis?
- Beta-lactam antibiotics.
- Glycopeptides.
- Cycloserine.
- Bacitracin.
- Phosphonic acid derivatives.
What biological properties does Lipid A possess as part of LPS?
- Toxicity (toxicitas) — Lipid A is an endotoxin responsible for the toxicity of lipopolysaccharide.
- Release (liberatio) — toxicity is manifested when the bacterial cell is destroyed by antibiotics and LPS is released into the medium.
- Pathogenetic significance (pathogenesis) — most manifestations of the biological activity of Gram-negative bacteria are associated with Lipid A.
What are protoplasts, spheroplasts, and L-forms of bacteria?
- Protoplasts (protoplasti) — bacteria completely devoid of a cell wall, usually spherical in shape, originating from Gram-positive microorganisms.
- Spheroplasts (spheroplasti) — bacteria with a partially preserved cell wall, formed from Gram-negative species.
- L-forms (formae L) — bacteria of the sphero- or protoplast type that have lost the ability to synthesize peptidoglycan but retain the capacity to multiply under the action of antibiotics or mutations.
Which bacteria naturally lack a cell wall?
- Mycoplasmas (Mycoplasma) — bacteria of the class Mollicutes (phylum Tenericutes), characterized by a complete absence of a cell wall and the presence of a cytoplasmic membrane containing sterols.
- Natural resistance (resistentia) — the absence of a cell wall causes absolute resistance of mycoplasmas to drugs targeting cell wall synthesis (penicillins, cephalosporins).
In Gram(+) bacteria, the wall is dense (high peptidoglycan content) and simple (without an outer membrane). In Gram(-) bacteria, it is thin, but features toxic lipopolysaccharide (LPS) in the outer membrane.
Class and exam questions
What is the uniqueness of the chemical composition of the bacterial cell wall?
Its composition includes components absent in humans and animals: N-acetylmuramic acid, D-isomers of amino acids (e.g., D-alanine), and meso-diaminopimelic acid.
Why are peptidoglycan synthesis enzymes a target for antibiotics?
Since humans lack the biochemical components of the bacterial cell wall, drugs that block its synthesis exhibit no toxicity to host cells.
How does lysozyme affect the cell wall?
Lysozyme (acetylmuramidase) cleaves the glycosidic bonds between sugars in the glycan backbone of peptidoglycan, depriving the cell of its rigid framework.
What is the difference between tetrapeptide cross-links in Gram-positive and Gram-negative bacteria?
In Gram-negative bacteria, the chains are joined by a direct peptide bond. In Gram-positive bacteria, they are cross-linked via an interpeptide bridge (e.g., pentaglycine), creating a more complex spatial structure.
What else to study on this topic
- Mechanisms of bacterial adhesion to host cells involving teichoic acids
- Peptidoglycan biosynthesis as a target for antibacterial drugs
- The role of lipopolysaccharide (LPS) and Lipid A in the infectious process
- Outer membrane porin channels and their role in substance transport
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