Differences between Exotoxins and Endotoxins

Bacterial toxins are broadly divided into two general categories: exotoxins and endotoxins. Exotoxins are proteins produced inside pathogenic bacteria and are secreted into the surrounding medium, whereas endotoxins are an integral part of the bacterial cell wall and are released only during the lysis of bacteria.

The symptoms caused by endotoxin of gram-negative bacteria are similar (though vary in severity) to one another. In contrast, symptoms caused by exotoxins of different bacteria are usually different. For example, a strain of Escherichia coli that produces one type of exotoxin causes watery (non-bloody) diarrhea, whereas a different strain of E.coli that produces another type of exotoxin causes bloody diarrhea.

Exotoxins

Properties of Exotoxins

Exotoxins are polypeptides 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), and cytotoxin.

Cytotoxin

Cytotoxin acts to disrupt the structure of individual cells. Destroyed cells slough from the surface of the mucosa, leaving it raw and unprotected, which causes loss of secretory and absorptive functions of these cells. Further damage to these cells occurs due to a strong inflammatory response from the host.

Some cytotoxin acts on the gastric epithelial cells causing dysentery (stool contains numerous PMNs, and blood), and pain, cramps, and tenesmus are common symptoms. A cytotoxin produced by Staphylococcus aureus named Panton-Valentine leukocidin (PVL) causes leukocyte destruction and tissue necrosis.

Examples of cytotoxins-producing organisms are Clostridioides difficile, E.coli O157: H7, Staphylococcus aureus, Bacillus cereus, Haemophilus ducreyi, and Bordetella pertussis

Enterotoxin

Enterotoxin causes food poisoning characterized by prominent vomiting and watery, non-bloody diarrhea. They act primarily in the jejunum and upper ileum, where most fluid transport takes place. Enterotoxins alter the metabolic activity of intestinal epithelial cells, causing an outpouring of electrolytes and fluid into the lumen, resulting in profuse and watery diarrhea. A classical example of enterotoxin is the cholera toxin.

 Some enterotoxins act as a superantigen within the gastrointestinal tract to stimulate the release of large amounts of cytokines, stimulating the enteric nervous system to activate the vomiting center in the brain. Most enterotoxins are heat-resistant and are therefore usually not inactivated by brief cooking. Enterotoxins are also resistant to stomach acid and enzymes in the stomach and jejunum.

Enterotoxin producing Organisms

1. Clostridium perfringens
2. Staphylococcus aureus (staphylococcal enterotoxin)
3. Escherichia coli (heat-labile toxin and heat-stable toxin)
4. Escherichia coli (shiga toxin or verotoxin)
5. Vibrio cholerae (cholera toxin)
6. Bacillus cereus

Neurotoxins

Botulinum is the most potent neurotoxin produced by Clostridium botulinum. The toxin prevents the release of the neurotransmitter acetylcholine at the synapse of the neuromuscular junction, causing flaccid paralysis.

Tetanus toxin is a neurotoxin that prevents the release of inhibitory neurotransmitters involved in muscle relaxation. Muscle spasms and spastic paralysis occur when the inhibitory neurons are nonfunctional, and the excitatory neurons are unopposed.  

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;

• A or active subunit possesses toxic activity, and
• B, or binding subunit, is responsible for binding the exotoxin to specific receptors on the human cell membrane. The binding of the subunit B determines the specificity of the exotoxin.  For example, botulinum toxin acts at the neuromuscular junction because the subunit B binds to specific receptors on the surface of the motor neuron at the junction.

Important exotoxins that have A-B subunit structure include botulinum toxin, cholera toxin, diphtheria toxin, enterotoxin of E.coli, and tetanus toxin.

Important Bacterial Exotoxins and their Mechanism of Action

Name of Toxin	Mode of Action
Bacillus anthracis toxin	Edema factor is an adenylate cyclase; the lethal factor is a protease that cleaves MAP kinase, which is required for cell division.
Botulinum toxin	It is a protease; it blocks the release of acetylcholine by proteolytic cleavage of releasing proteins.
C. difficile toxin	Exotoxin A and B inactivate GTPases by glucosylation.
Cholera toxin	Stimulates adenylate cyclase by ADP-ribosylation
Clostridium perfringens toxins	Alpha toxin is lecithinase; enterotoxin is a superantigen.
Diphtheria toxin	Inactivates EF-2 by ADP-ribosylation
Enterotoxin of Escherichia coli	Labile toxin stimulates adenylate cyclase by ADP-ribosylation; stable toxin stimulates guanylate cyclase.
Enterotoxin of S. aureus	It is a superantigen acting locally in the gastrointestinal tract
Erythrogenic toxin (streptococcal pyrogenic exotoxins)	It is a superantigen of S. pyogenes. Its action is similar to toxic shock syndrome toxin of S. aureus.
Pertussis toxin	Stimulates adenylate cyclase by ADP-ribosylation; inhibits chemokine receptor.
Scalded skin syndrome toxin of S. aureus	It is a protease that cleaves desmoglein in desmosomes
Shiga toxin (Shigella dysenteriae and E. coli)	Shiga toxin inhibits protein synthesis in enterocytes by removing adenine from 28S ribosomal RNA.
Tetanus toxin	Blocks release of inhibitory neurotransmitter glycine by proteolytic cleavage of releasing proteins.
Toxin shock syndrome toxin (TSST) of S. aureus	It is a superantigen. It binds to class II MHC protein and T-cell receptor; induces IL-1 and IL-2.

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.

Properties of Endotoxins

1. Endotoxins are integral parts of the cell walls of gram-negative rods and cocci, in contrast to exotoxins, which are actively released from the cell. Endotoxins (LPS) are released from the surface of gram-negative bacteria in small pieces of the outer membrane.
2. Endotoxins are lipopolysaccharides (LPS), whereas exotoxins are polypeptides.
3. The enzymes that produce LPS are encoded by genes on the bacterial chromosomes rather than plasmid or bacteriophage DNA, which usually encodes the exotoxins.
4. The toxicity of endotoxins is low in comparison with that of exotoxins. 
5. Though variations are seen in effectiveness or severity, all endotoxins produce the same generalized effects of ever and shock. The endotoxins of gram-negative bacteria are the best-established causes of septic shock. Septic shock is one of ICU’s leading causes of death and has an estimated mortality rate of 30% to 50%.
6. Endotoxins are weakly antigenic; they induce protective antibodies so poorly that multiple episodes of toxicity can occur.
7. No toxoids have been produced from endotoxins, and endotoxins are not used as antigens in any available vaccine.

Effects of Endotoxins

The biological effects of endotoxin include activation of macrophage, fever, hypotension/shock, disseminated intravascular coagulation (DIC), and complement activation.

1. Fever is due to the release of IL-1 (endogenous pyrogen) and IL-6, which act on the hypothalamic temperature-regulatory center). IL-1 and IL-6 are synthesized and released by macrophages. The macrophage is the major site of action of endotoxin.
2. Hypotension, shock, and impaired perfusion of essential organs due to nitric oxide-induced vasodilation, TNF-induced increased capillary permeability, bradykinin-induced vasodilation, and increased permeability of capillaries.
3. Endotoxin activates macrophages and increases their phagocytic ability. Endotoxin is also a polyclonal activator of B cells (but not T cells), thus increasing the production of antibodies.
4. Endotoxin also activates the alternate pathway of the complement cascade. This results in inflammation and tissue damage.
5. Endotoxins may also activate a coagulation cascade causing disseminated intravascular coagulation (DIC). DIC leads to the formation of petechiae and purpura, thrombosis, and tissue ischemia, resulting in the failure of vital organs.

Differences between Exotoxins and Endotoxins

Though various similarities exist between exotoxins and endotoxins, they differ in many aspects. For example, both exotoxins and endotoxins alone can cause symptoms without bacteria in the host. A person eating food containing preformed exotoxin can get botulism so does the person containing pyrogenic substances in the intravenous solution.

The major differences between exotoxins and endotoxins are tabulated here;

Property	Exotoxins	Endotoxins
Biomolecule	Proteins	Lipopolysaccharide-lipoprotein complex
Location of genes	Plasmid or bacteriophage	Bacterial chromosome
Source	Exotoxins are released 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 toxins with formaldehyde will destroy toxicity, but treated toxins remain immunogenic. Toxids are used as vaccines.	No toxoid formed, and no vaccine is available
Typical disease	Tetanus, diphtheria, botulism	Meningococcemia, sepsis by gram negative rods

References and further readings

1. Levinson Warren et.al. Review of Medical Microbiology and Immunology. A guide to clinical infectious diseases. MC Graw Hill Education. LANGE