Architecture and Outer Membrane
Gram-negative bacteria possess a unique cell envelope structure. Unlike Gram-positive microorganisms, their peptidoglycan layer is very thin. Overlying this layer is a critical structural component—the outer membrane.
These two layers are securely linked by specialized molecules called lipoproteins. Under an electron microscope on an ultrathin section, the outer membrane appears as a characteristic undulating trilaminar structure. Physically, it is a lipid bilayer that closely resembles the standard cytoplasmic cell membrane.
However, the outer membrane of Gram-negative microbes has a distinctly asymmetrical structure. It is a mosaic system composed of lipids and proteins. Its inner leaflet is formed by standard phospholipids, whereas the outer leaflet consists entirely of a specific component—lipopolysaccharide (LPS).
Lipopolysaccharide (LPS): Structure and Role
The lipopolysaccharide molecule is a key antigen and a potent virulence factor of the bacterium. It has a complex three-component structure, with each fragment serving a specific function:
- Lipid A. This is the core, highly conserved part of the molecule that acts as an "anchor" deeply embedded in the membrane. Lipid A functions as an endotoxin. It is critical to understand that its toxic effect is not expressed by the intact bacterium, but only when the cell wall is disrupted (e.g., by bactericidal antibiotics) and LPS is released into the environment, which can lead to severe endotoxic shock in the patient.
- Core Polysaccharide. A relatively constant oligosaccharide that serves as a bridge between lipid A and the outer polysaccharide chain. The core always contains a specific sugar derivative—ketodeoxyoctonate (KDO).
- O-antigen (O-specific chain). This is a long polysaccharide chain directed toward the external environment. This fragment is highly variable. The O-antigen determines the serogroup and serotype of a specific bacterium, which is used in laboratories for precise strain identification via specific antisera.
Interestingly, the completeness of the LPS molecule directly affects colony morphology upon plating. Bacteria with a complete, long LPS form smooth and shiny colonies (S-form, for smooth). If genetic mutations shorten the LPS molecule (causing a defect), the colonies appear rough (R-form, for rough).
Outer Membrane Proteins
The outer membrane performs vital barrier and transport functions thanks to specialized matrix proteins that span it entirely.
Key transport structures are porins. These proteins aggregate to form open hydrophilic channels (pores) in the membrane. Their primary role is to facilitate the passive transport of water and small hydrophilic molecules from the external environment into the cell. Porin permeability is strictly limited: only substances with a molecular weight not exceeding 700 Da can pass through these channels.
Periplasmic Space
An isolated compartment—the periplasmic space (periplasm)—is located between the inner (cytoplasmic) and outer membranes of a Gram-negative bacterium.
This region is filled with a solution containing transport proteins (components of substance-transfer systems) and a wide array of enzymes. Periplasmic enzymes include various proteases, lipases, phosphatases, and nucleases.
The presence of $\beta$-lactamases in the periplasmic space is of paramount clinical importance. These enzymes can destroy antibiotic molecules before they reach their intracellular targets, representing one of the primary mechanisms of bacterial resistance to antibiotic therapy.