In gram-positive bacteria, the cell wall consists of a thick peptidoglycan layer, totaling $${}30-100$$ nm in thickness. Within this thick layer, the N-acetylmuramic acid (NAM) molecules are connected by tetrapeptide chains linked via pentaglycine cross-bridges. This differs from gram-negative bacteria, where the tetrapeptide chains extending from each NAM unit are directly cross-linked. The peptidoglycan layers in gram-positive bacteria are commonly embedded with teichoic acids (TAs) on the external side, which increase the rigidity of the cell wall.

Gram Positive Cell Walls

Gram-negative cells possess a cell envelope with a significantly thinner layer of peptidoglycan—no more than about $${}4$$ nm thick—compared to gram-positive cells. Their overall cell envelope structure is highly complex, featuring a periplasmic space and an outer membrane located external to the peptidoglycan layer.

Gram Negative Cell Walls

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Peptidoglycan, also referred to as murein, is the major component of bacterial cell walls and is uniquely found in bacteria. Structurally, it resembles a fabric-like meshwork layer composed of long chains of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) molecules. These chains possess significant two-dimensional tensile strength because peptide bridges connect the NAG and NAM units within each peptidoglycan layer.

Peptidoglycan

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The structural integrity of the bacterial cell wall relies on the cross-linking of tetrapeptide chains that extend from each N-acetylmuramic acid (NAM) unit within the peptidoglycan layer. The nature of this connection differs between bacterial types: in gram-negative bacteria, the tetrapeptide chains are directly cross-linked to one another, whereas in gram-positive bacteria, these tetrapeptide chains are connected by pentaglycine cross-bridges.

Peptidoglycan Cross-Linking

The assembly of peptidoglycan is a multi-step process that determines the shape of a bacterial cell. The individual subunits of peptidoglycan are synthesized inside the bacterial cell. These subunits are subsequently exported across the membrane and assembled into structured layers on the cell's exterior.

Peptidoglycan Assembly

Antibiotic medications, such as penicillin and its derivatives, target bacterial cells by interfering with the synthesis of their peptidoglycan cell walls. Because humans and all other eukaryotes do not possess peptidoglycan cell walls, these antibiotics can selectively eliminate bacteria without harming host cells.

Antibiotic Interference with Peptidoglycan

Certain cells of the human immune system can identify bacterial pathogens by detecting the peptidoglycan present on their cell surfaces. Once recognized, these immune cells engulf the bacterial cell and employ specific enzymes, such as lysozyme, to break down and digest the peptidoglycan in the cell wall, ultimately destroying the pathogen.

Immune Recognition of Peptidoglycan

Archaeal cell wall structure differs from bacterial cell walls primarily because archaea do not contain peptidoglycan. Instead, they often contain a similar polymer called pseudopeptidoglycan (or pseudomurein). Alternatively, some archaea possess a layer of glycoproteins or polysaccharides that serves as the cell wall, and a few archaeal species appear to lack cell walls entirely.

Archaeal Cell Walls vs. Bacterial Cell Walls

Amoxicillin is a commonly prescribed penicillin derivative that acts as an antibiotic by targeting the peptidoglycan of bacterial cell walls. If an infection does not resolve after a full course of amoxicillin, it suggests that the causative agent likely lacks peptidoglycan (such as a virus, a fungus, or a bacterium lacking this structure) or that the pathogenic bacterium has developed antibiotic resistance.

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