outer membrane
Summary
The outer membrane is a distinguishing structural feature of Gram-negative bacteria, located external to the peptidoglycan layer. It contains lipopolysaccharide (LPS/endotoxin), porins, and phospholipids, contributing to antibiotic resistance and immune evasion. Its presence (or absence) is fundamental to Gram staining and antibiotic selection.
Detail
The outer membrane is a lipid bilayer unique to Gram-negative bacteria, situated outside the thin peptidoglycan layer, which itself sits in the periplasmic space. It is asymmetric: the inner leaflet is composed of phospholipids, while the outer leaflet contains lipopolysaccharide (LPS), a potent endotoxin composed of Lipid A (the toxic component that activates TLR4/CD14 on immune cells, triggering septic shock via cytokine release—TNF-α, IL-1, IL-6), core polysaccharide, and O antigen (used for serotyping and evading antibody recognition due to antigenic variation).
Embedded in the outer membrane are porins (e.g., OmpF, OmpC), which are water-filled channels allowing passive diffusion of small hydrophilic molecules, including some antibiotics. Mutations or downregulation of porins is a mechanism of antibiotic resistance (e.g., to beta-lactams and fluoroquinolones). The outer membrane also contains various proteins involved in nutrient transport, adhesion (fimbriae/pili anchoring), and secretion systems (Type III, Type IV) used to inject virulence factors into host cells.
Clinically, the outer membrane explains why Gram-negative bacteria are inherently more resistant to certain antibiotics (e.g., vancomycin, which is too large to cross porins) and detergents/dyes than Gram-positive bacteria. It also explains the endotoxic shock syndrome seen in Gram-negative sepsis (e.g., E. coli, Neisseria meningitidis, Pseudomonas aeruginosa)—characterized by fever, hypotension, DIC, and shock due to LPS-mediated immune activation.
The outer membrane is disrupted by polymyxins (colistin, polymyxin B), which act as cationic detergents binding LPS, useful against multidrug-resistant Gram-negative organisms. EDTA can also destabilize the outer membrane by chelating divalent cations (Mg2+, Ca2+) that stabilize LPS-LPS interactions.
Key boards associations: LPS/endotoxin is heat-stable, not inactivated by formaldehyde (unlike exotoxins used in toxoid vaccines), and induces fever via IL-1/TNF-α-mediated hypothalamic prostaglandin synthesis. The Limulus amebocyte lysate assay detects endotoxin.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- Lippincott Illustrated Reviews: Microbiology
- Robbins Basic Pathology
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