Cell wall of Gram-negative bacteria

Architecture and Outer Membrane

Gram-negative bacteria possess a unique envelope structure. Unlike Gram-positive microorganisms, their peptidoglycan framework is very thin. Overlying it is a crucial structural component—the outer membrane.

These two layers are securely connected to each other by special molecules called lipoproteins. When viewed on an ultrathin section of a bacterium under an electron microscope, the outer membrane appears as a characteristic wavy trilaminar structure. Physically, it is a lipid bilayer that externally closely resembles a typical cell cytoplasmic 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 layer consists entirely of a specific component—lipopolysaccharide (LPS).

Lipopolysaccharide (LPS): Structure and Role

The lipopolysaccharide molecule is a key antigen and a potent bacterial pathogenicity factor. It has a complex three-component structure, with each fragment performing a specific function:

1. Lipid A. This is the basic, highly conserved part of the molecule that acts as an "anchor" securely embedded in the thickness of the membrane. Lipid A is precisely the component that functions as an endotoxin. It is important to understand that its toxic effect does not manifest in an intact bacterium, but only upon the destruction of the cell wall (e.g., under the action of antibiotics) and the release of LPS into the environment. This can lead to the development of severe endotoxic shock in the patient.
2. Core (core region). A relatively constant (conserved) oligosaccharide that serves as a linking bridge between lipid A and the outer polysaccharide chain. The core necessarily contains a specific molecule—ketodeoxyoctonic acid (KDO).
3. O-specific chain (O-antigen). This is a long polysaccharide strand directed toward the external environment. This fragment is highly variable. The O-antigen determines the serogroup and serovar of a specific bacterium, which is used in laboratories for the precise identification of strains using immune sera.

Interestingly, the integrity of the LPS molecule directly affects colony morphology upon plating. Bacteria with a complete, long LPS form smooth and shiny colonies (S-forms, from smooth). Conversely, if the LPS molecule is shortened due to genetic mutations (a defect arises), the colonies acquire a rough appearance (R-forms, from rough).

Outer Membrane Proteins

The outer membrane performs a vital barrier and transport function thanks to specialized matrix proteins that span it entirely.

Key transport structures are porins. These proteins oligomerize and form open hydrophilic channels (pores) in the membrane. Their main task is to ensure the passive transport of water and small hydrophilic molecules from the external environment into the cell. The throughput capacity of porins is strictly limited: only substances with a molecular weight not exceeding 700 Da can pass through such channels.

Periplasmic space

An isolated zone—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 range of enzymes. Periplasmic enzymes include various proteases, lipases, phosphatases, and nucleases.

Of immense importance to clinical medicine is the presence of $eta$-lactamases in the periplasmic space. These enzymes are capable of destroying antibiotic molecules even before they reach their targets inside the cell, which is one of the main factors of bacterial resistance to antibacterial therapy.

Class and exam questions

What else to study on this topic

• Pathogenesis of endotoxic shock: mechanisms of action of lipid A
• Structural features of peptidoglycan in Gram-negative bacteria
• Resistance mechanisms: classification and function of $\beta$-lactamases
• Serological identification of bacteria by O-antigen
• Mutation genetics: how the transition from S-form to R-form occurs

Other topics in the section "Morphology and Classification of Microbes"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.