Bacterial toxins are broadly divided into two general categories: exotoxins and endotoxins.
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 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
|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|
Acharya TankeshwarHello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.
Flagella: Structure, Arrangement, Function
Flagella are long, whiplike appendages that move the bacteria toward nutrients and other attractants
Cell Wall–Deficient Bacteria
Although most prokaryotes cannot survive in nature without their cell walls, some do so naturally. For example, mycoplasma and thermoplasma.