Eubacteria, or “true bacteria,” are single-celled prokaryotes belonging to the domain Bacteria. They lack a membrane-bound nucleus and complex internal structures. A defining feature is the rigid cell wall, located just outside the plasma membrane. The cell wall provides mechanical strength, maintains the bacterium’s shape, and protects the cell from bursting. This protection is necessary because the cell’s cytoplasm has a high concentration of dissolved substances, creating internal pressure that the wall must counteract to prevent osmotic rupture.
The Core Component: Peptidoglycan
The material that makes up the Eubacteria cell wall is a complex polymer known as peptidoglycan, or murein. This substance is a large, mesh-like macromolecule composed of repeating sugar units and short peptide chains. The sugar backbone consists of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) molecules linked in long strands.
Attached to each NAM molecule is a short chain of amino acids, the peptide stem. The structural integrity of the cell wall depends on cross-linking, where the peptide stem of one strand links to a peptide stem on an adjacent strand. This bonding creates a vast, three-dimensional network that completely surrounds the cell, providing immense tensile strength.
The density of this mesh allows the cell to withstand the high internal turgor pressure created by the cell’s concentrated contents. The chemical composition of the carbohydrate backbone is highly conserved across nearly all Eubacteria, though the exact amino acids and the method of cross-linking can vary slightly between different bacterial species.
Structural Differences in Bacterial Walls
While peptidoglycan is the universal building block, its organization leads to two major structural types, which are distinguished by the Gram stain technique.
Gram-Positive Bacteria
Gram-positive bacteria possess a cell wall characterized by a remarkably thick, multi-layered sheath of peptidoglycan. This dense structure measures between 20 and 80 nanometers in thickness and makes up a significant proportion of the cell envelope’s dry weight. This single, thick layer sits directly outside the cell membrane.
Embedded within this thick peptidoglycan layer are anionic polymers called teichoic acids. These molecules help stabilize the wall. They are classified as lipoteichoic acids, which anchor the wall to the underlying plasma membrane, and wall teichoic acids, which are covalently linked to the peptidoglycan itself.
Gram-Negative Bacteria
Gram-negative bacteria display a more complex, multi-layered envelope structure. These cells have a much thinner layer of peptidoglycan, often only a few nanometers thick. This thin layer is located in the periplasm, a gel-like compartment between the two membranes that houses various proteins and enzymes.
The defining feature of Gram-negative bacteria is the presence of an outer membrane positioned external to the peptidoglycan layer. This outer membrane is unique because its external leaflet is primarily composed of lipopolysaccharides (LPS). LPS acts as a protective barrier, limiting the entry of harmful substances, including certain antibiotics and detergents.
Why the Cell Wall Matters Medically
The bacterial cell wall represents a prime target for therapeutic agents because this structure is unique to bacteria and is completely absent in human and animal cells. This difference allows certain drugs to interfere with bacterial processes without causing significant harm to the host. The most notable example of this strategy involves the beta-lactam class of antibiotics, which includes penicillin.
Penicillin works by specifically targeting the machinery responsible for assembling the peptidoglycan mesh. It binds to and inactivates bacterial enzymes called transpeptidases, also known as penicillin-binding proteins. These enzymes are necessary to form the peptide cross-links that provide the cell wall with its structural rigidity.
By preventing the formation of these cross-links, the antibiotic weakens the cell wall, particularly as the bacterium attempts to grow and divide. Without a structurally sound wall, the high internal osmotic pressure causes the cell to swell until its membrane ruptures, a process called lysis, effectively killing the bacteria.