Microbial Toxins
Secreted proteins: actively released by living bacteria (predominantly Gram positive, though some Gram negative organisms also produce exotoxins) into the surrounding environment or directly into host cells.
- Specific cellular targets: each exotoxin acts on a defined molecular target, producing a specific, predictable clinical effect, in contrast to the more generalised inflammatory effect of endotoxin (below).
- High potency: exotoxins are typically active at very low concentrations, among the most potent biological poisons known (e.g. botulinum toxin, tetanus toxin).
- Toxoid formation: some exotoxins can be chemically or heat-inactivated while retaining their antigenic structure, producing a toxoid, immunogenic but nontoxic, the basis of tetanus and diphtheria vaccines.
Major functional categories:
| Category | Mechanism | Examples |
|---|---|---|
| A-B toxins | Two-subunit structure: the B subunit binds a specific host cell receptor and mediates entry, the A subunit is the enzymatically active toxic component | Diphtheria toxin, cholera toxin, tetanus toxin, botulinum toxin, Shiga toxin |
| Membrane damaging toxins | Directly disrupt host cell membrane integrity, by pore formation or enzymatic degradation of membrane phospholipids | Alpha toxin (Clostridium perfringens), streptolysin O, alpha-haemolysin (S. aureus) |
| Superantigens | Bind directly to MHC class II and T cell receptor outside the normal antigen-binding groove, causing massive nonspecific T cell activation and a systemic cytokine storm | Toxic shock syndrome toxin-1 (S. aureus), staphylococcal enterotoxins, streptococcal pyrogenic exotoxins |
