Paper I
Question
Differences between exotoxin and endotoxin
Answer
| Feature | Exotoxin | Endotoxin |
|---|---|---|
| Source | Secreted by living bacteria (mostly Gram-positive, some Gram-negative) | Structural component (lipid A of LPS) of the Gram-negative outer membrane |
| Chemical nature | Protein | Lipopolysaccharide (lipid A + core polysaccharide + O antigen) |
| Release | Actively secreted during bacterial growth | Released on bacterial lysis/death (also in small amounts during growth) |
| Heat stability | Generally heat-labile (destroyed by heat, except a few like staphylococcal enterotoxin) | Heat-stable (withstands autoclaving) |
| Toxicity | Highly potent, active in very small doses | Less potent, requires larger amounts |
| Specificity of action | Highly specific (targets a particular cell type/receptor/mechanism, e.g., neurotoxin, enterotoxin, cytotoxin) | Non-specific, similar systemic effects regardless of source organism |
| Antigenicity/toxoid formation | Strongly antigenic; can be converted to a toxoid (formalin-inactivated, retains antigenicity, loses toxicity) for vaccines | Weakly immunogenic; cannot be converted to a toxoid |
| Fever production | Usually does not produce fever directly | Pyrogenic — produces fever via cytokine (IL-1, TNF) release |
| Examples | Diphtheria toxin, tetanus toxin, botulinum toxin, cholera toxin | LPS of E. coli, Salmonella, Neisseria meningitidis, and other Gram-negative bacteria (basis of Gram-negative septic shock) |
| Gene location | Often plasmid- or phage-encoded | Chromosomally encoded (integral part of the outer membrane) |