Eubacteria, often called true bacteria, are single-celled prokaryotic organisms. These microorganisms are distinguished by the presence of a rigid cell wall, which functions as a protective outer layer. This cell wall maintains the bacterium’s shape and ensures its survival against environmental challenges. Its composition sets eubacteria apart from other life forms.
The Core Component: Peptidoglycan
The defining material of eubacterial cell walls is peptidoglycan (murein). This macromolecule forms a strong, mesh-like network that encases the bacterial cell. It is composed of repeating disaccharide units: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These sugar derivatives are linked by β-(1,4)-glycosidic bonds, forming long glycan strands.
Attached to each N-acetylmuramic acid unit is a short peptide chain (three to five amino acids). These peptide chains are crucial for structural integrity, cross-linking with peptide chains from adjacent glycan strands. This cross-linking creates a robust, three-dimensional lattice structure. These peptides often include D-amino acids, uncommon in other biological proteins, providing defense against host enzymes that target L-amino acids.
Architectural Differences: Gram-Positive and Gram-Negative Walls
While peptidoglycan is a universal component, its arrangement and associated structures vary, classifying bacteria as Gram-positive or Gram-negative. Gram-positive bacteria possess a thick, multi-layered peptidoglycan wall, up to 90% of the cell wall’s dry weight and 15 to 80 nanometers thick. These extensive layers are often intertwined with teichoic acids (wall teichoic acids and lipoteichoic acids). Teichoic acids contribute to the cell wall’s negative charge and rigidity, influencing cell shape and ion binding.
In contrast, Gram-negative bacteria feature a much thinner peptidoglycan layer, typically only one to three layers thick and about 10 nanometers in thickness. This thin layer is situated within the periplasm, between an inner cytoplasmic membrane and an outer membrane. The outer membrane is a distinct lipid bilayer containing lipopolysaccharide (LPS), unique to Gram-negative bacteria. LPS is composed of three parts: Lipid A (an endotoxin), a core polysaccharide, and an O-antigen extending outwards from the cell surface. The outer membrane also contains protein channels called porins, which regulate molecule passage.
Why the Cell Wall Matters: Roles and Relevance
The eubacterial cell wall serves several vital functions for bacterial survival. It provides structural integrity and defines cell shape, acting as an exoskeleton. A primary role is to protect the bacterium from osmotic lysis, particularly in hypotonic environments where water rushes into the cell. Without the rigid cell wall to counteract internal pressure, water influx would cause the cell’s plasma membrane to burst.
Beyond structural support, the cell wall also functions as a barrier against harmful substances, especially in Gram-negative bacteria due to their outer membrane. The cell wall’s unique composition makes it a significant target for many antibiotics. For example, penicillin and other beta-lactam antibiotics interfere with peptidoglycan synthesis by inhibiting peptide chain cross-linking. This disruption weakens the cell wall, leaving bacteria vulnerable to osmotic lysis and easier for the immune system to eliminate. Since human cells do not possess peptidoglycan, these antibiotics selectively target bacterial cells without harming human cells.
Comparing Eubacterial Cell Walls to Other Organisms
The cell wall is a common feature across life forms, but its chemical makeup differs significantly among domains, highlighting distinct evolutionary paths. For instance, archaea, another domain of single-celled microorganisms, possess cell walls lacking peptidoglycan. Instead, some archaea have cell walls made of pseudopeptidoglycan (structurally similar but with different sugars), while others feature S-layers (surface-layer proteins), glycoproteins, or polysaccharides as their primary wall components.
In eukaryotes, plants have cell walls composed primarily of cellulose, a complex carbohydrate made of glucose units. Plant cell walls also contain other polysaccharides like hemicellulose and pectin, providing additional structural support. Fungi have cell walls predominantly made of chitin, a polymer of N-acetylglucosamine, along with glucans and glycoproteins. These distinct compositions underscore the unique nature of the eubacterial cell wall, characterized by its peptidoglycan structure.