For most bacteria, the most significant boundary is the cell wall. This structure provides a necessary separation from the external environment, offering both protection and structural support. At the heart of the bacterial cell wall is a polymer known as peptidoglycan. This molecule, also called murein, is unique to bacteria and forms a mesh-like layer that encases the cell membrane. The integrity of this layer is so fundamental that its disruption can lead to the death of the bacterium.
Defining Peptidoglycan Structure
The strength of peptidoglycan lies in its intricate chemical makeup. It is a polymer composed of long chains of two alternating sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These sugar chains are arranged linearly, linked together by β-(1,4) glycosidic bonds, forming the carbohydrate backbone of the structure.
Attached to each NAM sugar is a short chain of amino acids, typically consisting of three to five residues. These peptide chains act as bridges, cross-linking adjacent sugar strands to one another. This cross-linking process creates a dense, three-dimensional mesh that envelops the entire bacterium, often referred to as the sacculus.
The Essential Functions of Peptidoglycan Cell Walls
The primary role of the peptidoglycan cell wall is to provide structural strength and maintain the distinct shape of the bacterial cell, whether it be spherical, rod-shaped, or spiral. This rigidity is necessary to counteract the high internal osmotic pressure, or turgor, that builds up within the cytoplasm. Without the support of the peptidoglycan layer, the delicate cytoplasmic membrane would be unable to withstand this pressure, causing the cell to swell and burst in a process called lysis.
The peptidoglycan sacculus acts as a protective barrier, shielding the cell from environmental threats and regulating the passage of molecules. It serves as a scaffold for anchoring other cell envelope components, such as proteins and teichoic acids. The structure is also dynamic, allowing for expansion and remodeling during cell growth and division.
Variations in Peptidoglycan: Gram-Positive and Gram-Negative Bacteria
Bacteria are classified into two groups, Gram-positive and Gram-negative, based on cell wall architecture and its reaction to the Gram stain. Gram-positive bacteria possess a thick, multi-layered peptidoglycan cell wall (30–100 nm thick). This dense layer accounts for up to 90% of the cell wall’s dry weight and retains the crystal violet dye used in the Gram stain, giving these bacteria a purple appearance.
In contrast, Gram-negative bacteria have a more complex cell envelope. They feature a thin peptidoglycan layer, only a few nanometers thick, constituting about 10% of their cell wall’s dry weight. This thin layer is situated in the periplasm, between the inner cytoplasmic membrane and a distinct outer membrane. During the staining process, the outer membrane is degraded by the decolorizing agent, and the thin peptidoglycan layer is unable to retain the crystal violet dye, resulting in a pink or red appearance after a counterstain is applied.
Peptidoglycan as a Key Target for Antibiotics
The unique presence of peptidoglycan in bacteria makes it an ideal target for antibiotics. Since human and other eukaryotic cells do not have this specific polymer, drugs that interfere with peptidoglycan synthesis can selectively kill bacteria with minimal harm to the host. These drugs work by preventing the proper assembly of the cell wall, ultimately leading to its failure and the death of the bacterial cell.
A prominent class of these drugs is the β-lactam antibiotics, which includes penicillin and its derivatives. These molecules work by inhibiting the enzymes, known as penicillin-binding proteins (PBPs), that are responsible for creating the peptide cross-links between the glycan chains. By blocking this final step in peptidoglycan synthesis, the antibiotic prevents the formation of a stable cell wall. Without a functional wall to resist internal turgor pressure, the bacterial cell undergoes lysis.