Bit · Micro
Endotoxin vs Exotoxin
Two ways bacteria damage their host. Endotoxin is the membrane itself releasing chaos when the bug dies. Exotoxin is a deliberate weapon the bug secretes.
Mechanism
The pivot is whether the toxin is structural or secreted:
- Endotoxin — lipopolysaccharide (LPS) is part of the outer membrane of gram-negative bacteria. Lipid A is the toxic moiety. Released when the bacterium lyses (or, to a lesser extent, during growth). Binds TLR4 on macrophages → IL-1, IL-6, TNF-α, nitric oxide → fever, hypotension, septic shock, DIC. Heat stable. Non-specific effects.
- Exotoxin — proteins secreted by both gram-positive and gram-negative bacteria. Each one has a specific mechanism and target (AB toxins, superantigens, pore-formers, etc.). Genes often encoded on plasmids or lysogenic phages. Heat labile (mostly). Highly potent in tiny doses. Many can be converted to toxoids for vaccines (diphtheria, tetanus).
Differentiator Table
| Endotoxin | Exotoxin | |
| Chemical nature | Lipopolysaccharide (lipid A is toxic moiety) | Polypeptide (protein) |
| Source | Outer membrane of gram-negative bacteria | Gram-positive and gram-negative; actively secreted |
| Released by | Bacterial lysis or death | Active secretion during growth |
| Heat stability | Stable (boiling does not destroy) | Labile (mostly destroyed at 60 °C; exception: staph enterotoxin) |
| Specificity of action | Non-specific — systemic inflammation | Specific — each toxin has a defined target and mechanism |
| Receptor / target | TLR4 / CD14 on macrophages → IL-1, IL-6, TNF-α, NO | Varies — examples: ADP-ribosylation, pore formation, superantigen |
| Potency | Low (μg doses) | High (ng doses; botulinum and tetanus among most potent known) |
| Antigenicity | Poor — does not induce strong neutralising antibody | Strong — induces antibodies; can be made into toxoids for vaccines |
| Vaccine availability | No | Yes — diphtheria toxoid, tetanus toxoid |
| Classic syndromes | Septic shock, DIC, fever (gram-negative sepsis) | Cholera (AB toxin), diphtheria (EF-2 inactivation), tetanus (GABA inhibition), botulism (ACh blockade), TSS (superantigen) |
The Pivot
Three questions tell you which one is at work:
- Gram-negative bacteremia with fever, hypotension, DIC? Endotoxin (LPS / lipid A).
- Specific toxic syndrome with a recognisable named mechanism (paralysis, watery diarrhoea, pseudomembranes, toxic shock)? Exotoxin.
- Is there a toxoid vaccine for it? If yes (tetanus, diphtheria) → exotoxin. There is no vaccine against LPS itself.
NBME-Style Stem
A 58-year-old man with cirrhosis develops fever, hypotension, and confusion. Blood cultures grow gram-negative rods. Laboratory studies show platelet count 22,000/mm³, prothrombin time prolonged, fibrinogen low. Which of the following is most directly responsible for his shock and coagulopathy?
Concept Anchor
Endotoxin is the bacterial cell wall itself — non-specific, gram-negative, heat-stable, drives sepsis. Exotoxin is a deliberately secreted protein — specific, often named for what it does, heat-labile, often vaccinable.