Eubacteria, often called true bacteria, are single-celled organisms ubiquitous in nearly every environment on Earth, from soil and water to the human body. A defining characteristic of eubacteria is their rigid cell wall, which encases the cell membrane. This complex outer layer is central to bacterial survival and function.
The Unique Building Block: Peptidoglycan
The primary component of the eubacterial cell wall is peptidoglycan (murein). This macromolecule forms a strong, mesh-like layer around the bacterial cytoplasmic membrane. Peptidoglycan is unique to bacteria and is not found in archaea or eukaryotic cells.
Peptidoglycan is a polymer composed of two alternating amino sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These sugar units are linked by beta-(1,4) glycosidic bonds, forming long glycan strands.
Attached to each N-acetylmuramic acid (NAM) unit is a short chain of three to five amino acids, called an oligopeptide or peptide stem. The strength and rigidity of the peptidoglycan layer come from the cross-linking of these peptide chains between adjacent glycan strands. This cross-linking, facilitated by enzymes like transpeptidases (penicillin-binding proteins), creates a robust, three-dimensional network. The specific amino acid sequence and the nature of these cross-links can vary among different bacterial species, contributing to variations in cell wall thickness and overall stability.
Different Designs: Gram-Positive and Gram-Negative Cell Walls
While peptidoglycan is a universal component of eubacterial cell walls, its arrangement and associated layers differ significantly between Gram-positive and Gram-negative bacteria. These structural distinctions form the basis of the Gram staining procedure, a common laboratory technique.
Gram-positive bacteria possess a thick peptidoglycan layer (20-80 nm), which can constitute 60-90% of their cell wall’s dry weight. This extensive, multi-layered peptidoglycan provides substantial structural integrity. Interwoven within this thick peptidoglycan are anionic polymers called teichoic acids and lipoteichoic acids. Teichoic acids, which can be covalently bound to peptidoglycan or anchored in the cytoplasmic membrane, help maintain the cell wall’s rigidity, regulate ion passage, and play a role in bacterial adhesion and immune recognition.
In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer (7-8 nm), representing about 5-10% of their cell wall. This thin layer is located within a space called the periplasm, between the inner cytoplasmic membrane and an outer membrane. The outer membrane is a unique feature of Gram-negative bacteria and contains lipopolysaccharides (LPS), phospholipids, and proteins. LPS, with its lipid A component embedded in the outer membrane, contributes to the structural integrity of the outer membrane and acts as a barrier against harmful substances.
Why the Cell Wall Matters
The eubacterial cell wall is indispensable for bacterial survival and interaction with their environment. Its rigid, mesh-like structure provides mechanical strength and maintains the cell’s characteristic shape (rod-like, spherical, or spiral). This structural integrity resists physical stresses and maintains cellular form.
Beyond shape maintenance, the cell wall acts as a protective barrier, shielding the bacterium from harmful external agents. It prevents osmotic lysis, the bursting of the cell due to excessive water intake in hypotonic environments. The cell wall counteracts the internal pressure generated by the high concentration of solutes within the bacterial cytoplasm.
The cell wall’s unique composition also makes it a significant target in medicine. Many antibiotics, such as penicillin, interfere with the synthesis of peptidoglycan, thereby weakening the cell wall and making the bacteria vulnerable to osmotic lysis. Furthermore, components of the cell wall, like lipopolysaccharides in Gram-negative bacteria and teichoic acids in Gram-positive bacteria, can trigger immune responses in a host, influencing the course of bacterial infections.