The Gram-Positive Bacterial Cell Wall: Structure & Function
Understand the distinct structure of the Gram-positive cell wall, a feature that defines its staining, function, and vulnerability to common antibiotics.
The exterior of most bacterial cells features a protective layer known as the cell wall. This structure is fundamental to classifying bacteria into two broad categories, Gram-positive and Gram-negative, based on the composition of their respective walls.
Core Structural Components
The defining feature of the Gram-positive cell wall is its thick layer of peptidoglycan. This polymer, also called murein, forms a mesh-like structure around the cytoplasmic membrane, providing structural strength. Peptidoglycan can constitute up to 90% of the Gram-positive cell wall, with numerous layers creating a wall that is 20 to 80 nanometers thick.
The peptidoglycan mesh is constructed from repeating chains of two alternating sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Extending from each NAM sugar is a short tetrapeptide chain. In many Gram-positive bacteria, these tetrapeptide chains are linked by a peptide interbridge, creating the rigid peptidoglycan sacculus.
Woven throughout the peptidoglycan layer are teichoic acids, which are unique to Gram-positive bacteria. There are two main types. Wall teichoic acids are covalently bonded to the peptidoglycan layer.
Lipoteichoic acids (LTAs) have a lipid component that anchors them to the underlying cytoplasmic membrane. These molecules extend from the membrane through the peptidoglycan layer to the cell surface. This arrangement secures the wall to the cell membrane, and both types of teichoic acids contribute to the negative charge of the cell surface.
The Gram Staining Process
The term “Gram-positive” originates from a diagnostic staining procedure developed by Hans Christian Gram. The process involves the sequential application of four different reagents to bacterial cells fixed on a microscope slide.
The procedure begins with crystal violet, a primary purple dye that stains all bacterial cells. Next, iodine is added, which acts as a mordant. The iodine forms large, insoluble complexes with the crystal violet dye within the cells.
A decolorization wash with an alcohol solution is the differentiating step. The thick peptidoglycan layer of Gram-positive bacteria dehydrates and shrinks, trapping the large crystal violet-iodine complexes inside the cell.
As a result, Gram-positive bacteria retain the initial purple dye. A final counterstain, safranin, is then applied but does not change their appearance. When viewed under a microscope, these cells appear purple or blue-violet.
Key Functions of the Cell Wall
The primary role of the Gram-positive cell wall is to maintain the cell’s shape and structural integrity. The rigid peptidoglycan layer provides the strength to withstand high internal turgor pressure. This prevents the cell from bursting in a hypotonic environment, a process known as osmotic lysis. This rigidity also confers characteristic shapes to bacteria, such as the spherical coccus or the rod-like bacillus.
The cell wall also serves as a scaffold for anchoring various surface proteins. These proteins perform a variety of functions, including acting as enzymes or as adhesins that allow the bacterium to attach to surfaces. Teichoic acids help organize these surface components.
The cell wall participates in cell growth and division. For a bacterium to divide, the existing cell wall must be enlarged and remodeled to form a septum, the new wall that divides the parent cell. This process is precisely regulated to ensure each new cell receives a complete cell wall.
Contrast with Gram-Negative Cell Walls
The Gram-positive cell wall stands in stark contrast to that of Gram-negative bacteria, primarily in the thickness of the peptidoglycan layer. Gram-negative bacteria possess a much thinner layer, often only a few nanometers thick, which is insufficient to retain the crystal violet-iodine complex during staining.
A defining feature absent in Gram-positive bacteria is the outer membrane. Gram-negative bacteria have a second, outer lipid bilayer that contains proteins and lipopolysaccharide (LPS). This outer membrane makes Gram-negative bacteria less permeable to certain substances.
Because of these structural differences, other components are also distinct. Teichoic and lipoteichoic acids are absent in Gram-negative bacteria. The periplasmic space between the cytoplasmic membrane and the peptidoglycan layer is also much smaller in Gram-positive bacteria.
Medical and Clinical Significance
The structure of the Gram-positive cell wall makes it a target for many antibiotics. Since peptidoglycan is not found in human cells, drugs can attack this structure without harming the host. The beta-lactam class of antibiotics, including penicillin, functions by inhibiting the enzymes that build the peptide cross-links in peptidoglycan, which weakens the wall and leads to cell lysis.
Cell wall components also play a direct role in how Gram-positive bacteria cause disease. Molecules such as teichoic acids and surface proteins can function as virulence factors. For example, they can act as adhesins, enabling bacteria to bind to host cells and tissues, which is a first step in establishing an infection.
Fragments of the cell wall, including peptidoglycan and lipoteichoic acids, can be recognized by the host’s immune system. These molecules are known as pathogen-associated molecular patterns (PAMPs) and can trigger a strong inflammatory response. This inflammation, while intended to fight infection, can contribute to the symptoms and tissue damage seen in bacterial diseases.