Types of Bacterial Cell Walls

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.

Amoxicillin

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.

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The cell wall is a structure found in most prokaryotes and some eukaryotes that envelopes the cell membrane. Working in combination with cytoskeletal elements, the cell wall maintains the cell's morphology and protects it from changes in osmotic pressure.

Cell Wall

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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The cell wall cannot be observed through light microscopy or simple stains. It may be demonstrated by microdissection, reaction with specific antibodies, mechanical rupture of the cell, differential staining procedures or by electron microscopy.

Viewing the Cell Wall

The cell wall is composed of peptidoglycan, a mucopeptide (formed by N acetyl glucosamine & N acetyl muramic acid) linked to peptide chains. Teichoic acids are embedded into the cell wall and may be linked to lipids (Lipoteichoic acid). These Lipotechoic acids connect peptidoglycan to the cytoplasmic membrane and gives the cell wall its rigidity. 

Composition of the Cell Wall

Peptidoglycan

The cell walls of some eukaryotic cells—such as plants, fungi, algae, and certain protists—provide structural stability and protection from environmental stresses like desiccation, changes in osmotic pressure, and traumatic injury. The chemical composition of these walls varies: plants use cellulose; fungi use chitin; and algae and protists can utilize biogenic silica, calcium carbonate, agar, or carrageenan.

Eukaryotic Cell Wall

Bacteria are traditionally classified into gram-positive, gram-negative, and atypical groups based on their staining patterns, which are determined by their cell wall properties. Gram-positive bacteria have a thick peptidoglycan layer that retains stain, gram-negative bacteria have a thin peptidoglycan layer surrounded by an outer membrane, and atypical bacteria cannot be stained by standard Gram stain procedures.

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