exotoxin
Summary
Exotoxins are potent, secreted protein toxins produced by both Gram-positive and Gram-negative bacteria that cause disease by damaging host cells or disrupting cellular processes. They are highly antigenic, often with high toxicity even in small amounts, and can be targeted by vaccines (toxoids). Unlike endotoxin (LPS), exotoxins have diverse, specific mechanisms of action.
Detail
Exotoxins are heat-labile polypeptides actively secreted by living bacteria, encoded by plasmids, bacteriophages, or chromosomal genes. They are classified by mechanism: (1) ADP-ribosylating toxins (e.g., diphtheria toxin inhibits EF-2; cholera toxin activates Gs, increasing cAMP; pertussis toxin inhibits Gi); (2) superantigens that cause polyclonal T-cell activation and massive cytokine release (e.g., TSST-1, staphylococcal enterotoxins) leading to toxic shock syndrome; (3) pore-forming/membrane-damaging toxins (e.g., alpha-toxin of S. aureus, streptolysin O); (4) neurotoxins (botulinum toxin blocks ACh release causing flaccid paralysis; tetanus toxin blocks GABA/glycine release causing spastic paralysis); (5) toxins that inhibit protein synthesis (Shiga toxin, Shiga-like toxin cleave 28S rRNA, halting protein synthesis and causing hemorrhagic colitis/HUS). Exotoxins are highly immunogenic, inducing antibody responses, and can be inactivated (toxoided) with formaldehyde/heat to create vaccines (e.g., DTaP vaccine uses diphtheria and tetanus toxoids). Clinically, exotoxin-mediated diseases often present acutely and severely due to potent, specific molecular targets, distinguishing them from the more systemic, non-specific inflammatory response caused by endotoxin (LPS) from Gram-negative bacteria. High-yield boards testing points: know AB toxin mechanisms, associate toxins with their bacterial species and clinical syndromes, and differentiate exotoxin vs endotoxin properties (source, chemical composition, heat stability, antigenicity, toxicity, and vaccine potential).
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- Harrison's Principles of Internal Medicine
- Levinson's Review of Medical Microbiology and Immunology
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