Last updated on June 26th, 2021
Bacterial toxins are broadly divided into two general categories: exotoxins and endotoxins.
Exotoxins
They are polypeptide released extracellularly as the organism grows. Exotoxins may travel from a focus of infection to a distant part of the body and cause damage. E.g. neurotoxin (botulinum toxin, tetanus toxin), enterotoxin (cholera toxin), cytotoxin.
Exotoxins are produced by several Gram-positive and Gram-negative bacteria and are among the most toxic substances known. Many exotoxins have an A-B subunit structure; the A (or active) subunit possesses the toxic activity, and the B (or binding) subunit is responsible for binding the exotoxin to specific receptors on the membrane of the human cell.
Endotoxins:
Endotoxins are lipopolysaccharide, which is an integral part of the cell wall of Gram-negative bacteria. The name endotoxin is derived from the fact that these toxins are generally cell-bound and released only when the cell lyses.
Differences between Exotoxins and Endotoxins
| Property | Exotoxins | Endotoxins |
| Biomolecule/Chemistry | Proteins | Lipopolysaccharide-lipoprotein complex |
| Location of genes | Plasmid or bacteriophage | Bacterial chromosome |
| Source | Excreted by certain gram positive or gram negative bacteria | Cell wall of Gram-negative bacteria, released only after lysis of cells |
| Heat Stability | Destroyed rapidly at 60°C (except staphylococcal enterotoxin) | Stable at 100°C for one hour |
| Mode of Action (Symptoms) | Specific. Either cytotoxin, enterotoxin or neurotoxin with defined action on cells or tissues | General. Fever, diarrhea, vomiting |
| Toxicity | Highly toxic, often fatal (fatal dose on the order of 1 µg) | Weakly toxic, rarely fatal (fatal dose on the order of hundreds of micrograms) |
| Immunogenicity | Highly immunogenic, stimulate the production of neutralizing antibody (antitoxins) | Relatively poor immunogenicity |
| Toxoid potential/Vaccines | Treatment of toxin with formaldehyde will destroy toxicity, but treated toxins remain immunogenic.Toxids are used as vaccines. | No toxoid formed and no vaccine available |
| Typical disease | Tetanus, diphtheria, botulism | Meningococcemia, sepsis by gram negative rods |
