1. To develop an understanding of the role of exotoxins in infectious disease.
2. To emphasize the uniqueness of the bacterial exoenzymes.
3. To define the mode of action of bacterial exoenzymes.
Non-Membrane Damaging
1. Hyaluronidase - This is also called the spreading factor because it catalyzes the breakdown of hyaluronic acid, the substance that cements the human cells together. This allows the bacterial cells to spread through tissue causing a condition known as cellulitis.
2. Coagulase - This enzyme catalyzes the conversion of fibrinogen to fibrin with resultant clot formation.
3. Fibrinolysin - This catalyzes the conversion of plasminogen to the fibrinolytic enzyme plasmin. Thus it acts opposite of coagulase. In Staphylococcus aureus, the gene for fibrinolysin is on a bacteriophage and is expressed during lysogeny.
4. Lipase - Production of excessive amounts of lipase allow bacteria to penetrate fatty tissue with the consequent formation of abscesses.
5. IgA protease - Many bacteria which colonize the mucous membranes produce an IgA protease which degrades secretory IgA.
6. Collagenase - This enzyme catalyzes the degradation of collagen, a scleroprotein found in tendons, nails and hair.
Membrane Damaging
1. Hemolysins - There are many different types of hemolysins but, in each case, the end result is lysis of the red blood cell with resultant anemia.
2. Leukocidins - Again, there are many different types of leukocidins, and some are specific for only one type of leukocyte. However, the end result in lysis of leukocytes with resultant leukopenia.
3. Phospholipase - This enzyme attacks any cell with phospholipid in its membrane. The result is widespread cell lysis. Lecithinase - (phospholipase C) is an enzyme which breaks down the lecithin in the human cell plasma membrane, resulting in cell lysis. It is especially active on red blood cells. It is also called toxin.
These promote the breakdown of nicotinamide adenine dinucleotide (NAD) into nicotinamide and adenine diphosphate ribose (ADPR) and the covalent binding of the ADPR to various proteins, thus inactivating the bound protein.
1. Diphtheria toxin. Corynebacterium diphtheriae cells lysogenized with the ß-phage produce a diphtheria toxin which is a bipartite molecule, composed of a B subunit which mediates binding to a specific human cell surface receptor and an A subunit which possesses enzymatic (ADP-ribosyltransferase) activity. The substrate of the reaction is human elongation factor 2 (EF2), an essential part of the protein synthetic machinery.
diphtheria toxin
EF2 + NAD --------------------> ADPR-EF2 + nicotinamide + H+
The result of this reaction is inhibition of protein synthesis and cell death.
2. Pseudomonas aeruginosa exotoxin A. This works in the same manner as diphtheria toxin, i.e., it catalyzes the ADP-ribosylation of EF-2. Human epithelial cell death occurs.
Mode of action of exotoxin A. A. Exotoxin A, composed of fragments A and B, inhibits eukaryotic cell protein synthesis by binding to specific receptors in the cell membrane. B. After fragment B binds to a cell receptor, fragment A enters the cell. C. Fragment A catalyzes the binding of nicotinamide adenine dinucleotide (NAD) to Elongation Factor 2 (EF2), which is required for translocation of nascent polypeptide chains on eukaryotic ribosomes. D and E. The reaction terminates in the irreversible formation of an adenosine diphosphate ribose. EF2 diphosphate -- EF2 complex with the release of nicotinamide and hydrogen.
From Medical Microbiology, 2nd ed., Murray et al., p. 255, Fig. 27-1. Reproduced with permission.
3. Pseudomonas aeruginosa exoenzyme S. This is an ADP-ribosyl transferase whose substrate is unknown. However EF-2 is not the substrate. It also causes human epithelial cell death.
4. Cholera toxin. Vibrio cholerae growing in the intestine secretes an exotoxin composed of 5 B subunits, an A subunit and an A2 subunit. On exposure to small bowel epithelial cells, each B subunit binds to a receptor on the gut epithelium. Following binding the A and A2 moieties migrate through the epithelial cell membrane. The A subunit is an ADP-ribosyl transferase that catalyzes the transfer of ADPR from NAD to a guanosine triphosphate (GTP)-binding protein that regulates adenylate cyclase activity. The ADP-ribosylation of GTP binding protein inhibits the GTP turnoff reaction and causes a sustained increase in adenylate cyclase activity which results in excess secretion of isotonic fluid into the intestine with resulting diarrhea.
From Medical Microbiology, 2nd ed., Murray, et al., p. 242, Fig. 25-1. Reproduced with permission.
5. Labile toxin (LT) of Escherichia coli. This toxin is identical to that of cholera toxin. The ability to produce it is mediated by a plasmid.
6. Bordetella pertussis toxin. During an episode of whooping cough, the B. pertussis cell produces an exotoxin composed of an A portion and 4 B portions. The A subunit is an ADP-ribosyl transferase which elevates cAMP but in a way different from cholera toxin. It ribosylates a 41,000 MW membrane protein which specifically binds guanine nucleotide (the G1 protein).
Non-Ribosylating Toxins
1. Shiga toxin. Species of Shigella carry the gene for shiga toxin on the chromosome. This toxin has an A subunit and 5 B subunits. The A subunit can be divided into A1 and A2 subunits. The A1 moiety binds to the 60S human ribosome which inhibits protein synthesis. The toxin has a multiplicity of effects; it is neurotoxic, cytotoxic and enterotoxic.
2. Anthrax toxin. Bacillus anthracis produces an exotoxin composed of three distinct proteins: protective antigen, edema factor and lethal factor. The protective antigen is the binding protein, the edema factor is an adenyl cyclase and the lethal factor has an unknown function but is thought to be enzymatic. Dermal necrosis is the result of the toxin action.
3. Tetanus toxin. Clostridium tetani produces a non-enzymatic protein that blocks the release of cholinesterase at the myoneural junction and -aminobutyric acid at the neural-neural junction in the spinal cord. The result is muscle spasm.
Mechanism of tetanospasmin activity. A. Neurotransmission is controlled by the balance between excitatory and inhibitory neurotransmitters. B. The inhibitory neurotransmitters (e.g., GABA, glycine) prevent depolarization of the postsynaptic membrane and conduction of the electrical signal. C. Tetanospasmin does not interfere with production or storage of GABA or glycine, but rather their release (presynaptic activity). D. In the absence of inhibitory neurotransmitters, excitation of the neuroaxon is unrestrained.
From Medical Microbiology, 2nd ed., Murray, et al., p. 298, Fig. 32-5.
4. Botulinum toxin. Clostridium botulinum produces a non-enzymatic protein that blocks the release of acetylcholine at the myoneural junction. Muscle paralysis is the result.
From Medical Microbiology, 1990, Murray, et al., p 194, Figure 18-12. Reproduced with permission.
5. Pyocyanin. Pseudomonas aeruginosa produces this non-enzyme protein which binds to the flavoproteins of the cytochrome system. It interferes with terminal electron transport causing an energy deficit and cell death.
6. Adenylate cyclase. Bordetella pertussis produces a calmodulin-independent adenylate cyclase which inhibits and/or kills white blood cells.
7. NAD glycohydrolase. Shigella flexneri, upon being phagocytized, produces a NAD glycohydrolase which rapidly depletes the phagocyte of NAD, thus blocking cellular metabolism and bacterial cell killing.
Toxins with Undefined Mechanism of Action
1. Trachea toxin. Bordetella pertussis tracheal cytotoxin kills cilia-bearing cells.
2. ß-toxin. Clostridium difficile produces a beta toxin which causes a necrotic enteritis.
3. Exfoliating toxin. Staphylococcus aureus produces an exfoliating toxin which causes a sloughing of skin (scalded skin syndrome).
4. Toxic shock syndrome toxin. Staphylococcus aureus produces this toxin which has an undefined mode of action but is mediated through induction of interleukin-1. It causes hypotension, rash, fever and desquamation of skin.
5. Erythrogenic toxin. Streptococcus pyogenes produces this toxin which is similar to toxic shock syndrome toxin.
1. Cellulitis is caused by bacteria producing hyaluronidase, the spreading factor.
2. Clot formation is caused by bacteria producing coagulase.
3. Clot dissolution is caused by bacteria producing fibrinolysin.
4. Abscess formation is caused by bacteria producing lipase.
5. Bacterial resistance to IgA is mediated by bacterial production of IgA protease.
6. Degradation of hair, nails and tendons is promoted by bacteria producing collagenase.
7. Anemia may be the result of bacteria producing hemolysin or phospholipase.
8. Leukopenia may be due to bacteria producing leukocidins.
9. Many bacteria produce a toxin, ADP-ribosyl transferase, that promotes the transfer of adenine diphosphate ribose from nicotinamide adenine dinucleotide (NAD) to various proteins, thus inactivating those proteins. Toxins acting in this way include the diphtheria toxin, Pseudomonas aeruginosa exotoxin A and exoenzyme S, Cholera toxin, Escherichia coli labile toxin and whooping cough toxin.
10. Shiga toxin binds to the 60S portion of the human ribosome causing neurotoxic, cytotoxic and enterotoxic effects.
11. Anthrax toxin causes dermal necrosis.
12. Tetanus toxin blocks the release of cholinesterase at the myoneural junction, thus causing muscle spasm.
13. Botulinum toxin blocks the release of acetylcholine at the myoneural junction, thus causing muscle flaccidity.
14. Pyocyanin produced by P. aeruginosa binds to human cell flavoprotein, thus blocking electron transport via the cytochrome system.
15. Bordetella pertussis produces an adenylate cyclase which inhibits and/or kills white blood cells.
16. Shigella flexneri produces a NAD glycohydrolase which destroys human cell NAD, thus blocking cellular metabolism.
17. Staphylococcus aureus produces an exfoliating toxin and a toxic shock syndrome toxin which cause desquamation of skin.
18. Streptococcus pyogenes produces an erythrogenic toxin similar to the toxic shock syndrome toxin of S. aureus.
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