Bacteria are single-celled microorganisms found virtually everywhere, and their survival often depends on the integrity of their cell wall, a protective outer layer. This barrier not only provides mechanical strength but also determines how the bacterium interacts with its environment and with host organisms. The molecular composition of this wall is fundamental, serving as the basis for classifying bacteria into two large groups: Gram-positive and Gram-negative. Understanding the differences in these cell wall structures is essential for clarifying which specific molecules, such as lipopolysaccharides, a bacterium possesses.
The Defining Difference in Bacterial Cell Walls
The foundation for classifying bacteria stems from the Gram stain, a technique that visually separates species based on cell wall architecture. Gram-positive and Gram-negative bacteria differ primarily in the thickness and layering of their peptidoglycan, a unique polymer that forms a mesh-like layer providing structural support.
Gram-positive bacteria possess a thick, multi-layered peptidoglycan layer, which can constitute up to 95% of the cell wall material, situated directly outside the plasma membrane. In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer, typically 5% to 10% of the cell wall structure. This thin layer is sandwiched between the inner plasma membrane and a distinct outer membrane.
Lipopolysaccharides and the Gram-Negative Structure
Lipopolysaccharide (LPS) is found exclusively on the exterior surface of the outer membrane of Gram-negative bacteria. Its presence is directly tied to the existence of the outer membrane, as the lipid portion of LPS is embedded within this outer leaflet. LPS provides structural stability to the outer membrane and acts as a permeability barrier, protecting the cell from harmful external agents like detergents and antibiotics.
The molecule is a glycolipid composed of three distinct regions. The innermost section, called Lipid A, anchors the molecule into the outer membrane via multiple fatty acid chains. Lipid A is the biologically active portion, often referred to as an endotoxin, which triggers a potent inflammatory response in humans.
Extending outward from Lipid A is a short core oligosaccharide chain, followed by the highly variable O-antigen. This O-antigen is exposed to the environment and is a primary target for host immune recognition. Because Gram-positive bacteria lack an outer membrane, they cannot accommodate the LPS molecule in their cell wall structure.
What Gram-Positive Bacteria Have Instead
Since Gram-positive bacteria do not possess an outer membrane, they also do not have lipopolysaccharides. Instead, they incorporate specialized molecules into their thick peptidoglycan layer. The primary components fulfilling structural and immunological roles are teichoic acids (TA) and lipoteichoic acids (LTA). These acidic polymers are unique to Gram-positive species and can account for up to 50% of the cell wall material.
Wall teichoic acids are covalently linked directly to the peptidoglycan, while lipoteichoic acids are anchored to the plasma membrane via a lipid moiety. These molecules help regulate the cell wall’s negative charge and assist in cell division and maintenance. Like LPS in Gram-negative cells, LTA and TA interact with the host immune system, triggering an inflammatory response that can contribute to the symptoms of infection.