2. To emphasize the uniqueness of the bacterial exoenzymes.
3. To define the mode of action of bacterial exoenzymes.
- 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.
- 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
- Production of excessive amounts of lipase allow bacteria to penetrate
fatty tissue with the consequent
formation of abscesses.
protease - Many bacteria which colonize
the mucous membranes produce an IgA protease which degrades
6. Collagenase- This enzyme catalyzes the degradation of collagen, a scleroprotein found in tendons, nails and hair.
There are many different types of hemolysins but, in each case, the end
result is lysis of the red
blood cell with resultant anemia.
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.
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 a 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.
toxin. Corynebacterium diphtheriae
cells lysogenized with the b-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.
The result of this reaction is inhibition of protein synthesis and cell death.
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.
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.
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.
toxin (LT) of Escherichia coli.
This toxin is identical to that of cholera toxin. The ability to produce
mediated by a plasmid.
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).
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
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.
toxin. Clostridium tetani produces
an endopeptidase that cleaves synaptobrevins: this interferes with
vesicle formation at the myoneural junction and the neural-neural junction in the spinal cord. The result is muscle
spasm. The tetanus toxin serologically cross-reacts with the botulinum toxin.
toxin. Clostridium botulinum produces
an endopeptidase that blocks the release of acetylcholine at the
myoneural junction. Muscle paralysis is the result. the botulinum toxin, like tetanus toxin, cleaves synaptobrevin
thus interfering with vesicle formation. This toxin is used clinically in the treatment of dystonias.
aeruginosa produces this non-enzyme protein which binds to the flavoproteins
cytochrome system. It interferes with terminal electron transport causing an energy deficit and cell death.
pertussis produces a calmodulin-independent adenylate cyclase which
and/or kills white blood cells.
flexneri, upon being phagocytized, produces a NAD glycohydrolase which
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. b-toxin. Clostridium difficile produces a beta toxin which causes a necrotic enteritis.
Exfoliating toxin. Staphylococcus aureus produces an exfoliating
toxin which causes a sloughing of skin (scalded
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.
6. Mycolactone. Mycobacterium ulcerans produces this toxin which causes skin and muscle necrosis.
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.
Shigella flexneri produces a NAD glycohydrolase which destroys
human cell NAD, thus blocking cellular
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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