Previous Lecture  |  Syllabus  |  Next Lecture
Soluble Toxins



Table of Contents

While the bacterial cell may be composed of entities that are toxic; e.g., lipid A, a greater variety of toxins are synthesized and released from the cell in soluble form. These are the exotoxins.

Educational Objectives


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.

Specific (terms and concepts upon which you will be tested)

Extracellularly-Acting Toxins

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 a toxin.


ADP-ribosyl transferases

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 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.

                                                                   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

                            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.

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.

Cholera toxin

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

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 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.

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.

4. Botulinum 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.

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.   b-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.

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.

16.   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.

Previous Lecture  |  Syllabus  |  Next Lecture