Medical Bacteriology MBIO 460 Lecture 12 Dr. Turki Dawoud 2 nd Semester 1436/1437 H

Toxins 1.Exotoxins 2.Enterotoxin 3.Endotoxin

Tetanus and Botulinum Toxins Clostridium tetani and Clostridium botulinum, soil endospore-forming bacteria that occasionally cause disease in animals, produce potent AB exotoxins that affect nervous tissue. Neither species is very invasive, and virtually all pathogenic effects are due to neurotoxicity. C. botulinum sometimes grows directly in the body, causing infant or wound botulism, and also grows and produces toxin in improperly preserved foods (Section 37.6). Death from botulism is usually from respiratory failure due to flaccid muscle paralysis. C. tetani grows in the body in deep wound punctures that become anoxic, and although C. tetani does not invade the body from the initial site of infection, the toxin can spread via the neural cells and cause spastic paralysis, the hallmark of tetanus, often leading to death.

Botulinum toxin consists of seven related AB toxins that are the most potent biological toxins known. One milligram of botulinum toxin is enough to kill more than 1 million guinea pigs. Of the seven distinct botulinum toxins known, at least two are encoded on lysogenic bacteriophages specific for C. botulinum. The major toxin is a protein that forms complexes with nontoxic botulinum proteins to yield a bioactive protein complex. The complex then binds to presynaptic membranes on the termini of the stimulatory motor neurons at the neuromuscular junction, blocking the release of acetylcholine. Transmission of the nerve impulse to the muscle requires acetylcholine interaction with a muscle receptor and botulinum toxin prevents the poisoned muscle from receiving the excitatory signal (Figure 28.21). This prevents muscle contraction and leads to flaccid paralysis and death by suffocation, the outcome of botulism

Tetanus toxin is also an AB protein neurotoxin. On contact with the central nervous system, this toxin is transported through the motor neurons to the spinal cord, where it binds specifically to ganglioside lipids at the termini of the inhibitory interneurons. The inhibitory interneurons normally work by releasing an inhibitory neurotransmitter, typically the amino acid glycine, which binds to receptors on the motor neurons. Normally, glycine from the inhibitory interneurons stops the release of acetylcholine by the motor neurons and inhibits muscle contraction, allowing relaxation of the muscle fibers. However, if tetanus toxin blocks glycine release, the motor neurons cannot be inhibited, resulting in tetanus, continual release of acetylcholine, and uncontrolled contraction of the poisoned muscles (Figure 28.22). The outcome is a spastic, twitching paralysis, and affected muscles are constantly contracted. If the muscles of the mouth are involved, the prolonged contractions restrict the mouth's movement, resulting in the condition called lockjaw (trismus). If respiratory muscles are involved, prolonged contraction may result in death due to asphyxiation.

Both tetanus toxin and botulinum toxin both block release of neurotransmitters involved in muscle control, but the symptoms are quite different and depend on the particular neurotransmitters involved.

Enterotoxins Enterotoxins are exotoxins whose activity affects the small intestine, generally causing massive secretion of fluid into the intestinal lumen resulting in vomiting and diarrhea. Generally acquired by ingestion of contaminated food or water, enterotoxins are produced by a variety of bacteria, including the food-poisoning organisms Staphylococcus aureus, Clostridium perfringens, and Bacillus cereus, and the intestinal pathogens Vibrio cholerae, Escherichia coli, and Salmonella enteritidis. Cholera Toxin The enterotoxin produced by V. cholerae, the organism that causes cholera, is the best understood enterotoxin. Cholera is characterized by massive fluid loss through the intestines, resulting in severe diarrhea characterized by life-threatening dehydration and electrolyte depletion. The disease starts by ingestion of V. cholerae in contaminated food or water. The organism travels to the intestine, where it colonizes and secretes the cholera toxin. Cholera toxin is an AB toxin consisting of an A subunit and five B subunits.

In the gut, the B subunit binds specifically to GM1 ganglioside, a complex glycolipid found in the cytoplasmic membrane of intestinal epithelial cells (Figure 28.23). The B subunit targets the toxin specifically to the intestinal epithelium but has no role in alteration of membrane permeability; the toxic action is a function of the A chain, which crosses the cytoplasmic membrane and activates adenyl cyclase, the enzyme that converts ATP to cyclic adenosine monophosphate (cAMP). Figure The action of cholera enterotoxin.

cAMP is a mediator of many different regulatory systems in cells, including ion balance. The increased cAMP levels induced by the cholera enterotoxin induce secretion of chloride and bicarbonate ions from the mucosal cells into the intestinal lumen. This change in ion concentrations leads to the secretion of large amounts of water into the intestinal lumen (Figure 28.23). In acute cholera, the rate of water loss into the small intestine is greater than the possible reabsorption of water by the large intestine, resulting in massive net fluid loss. Cholera treatment is by oral fluid replacement with solutions containing electrolytes and other solutes to offset the dehydration-coupled ion imbalance. Expression of cholera enterotoxin genes ctxA and ctxB is controlled by toxR. The toxR gene product is a transmembrane protein that controls cholera A and B chain production as well as other virulence factors, such as the outer membrane proteins and pili required for successful attachment and colonization of V. cholerae in the small intestine.

Other Enterotoxins Enterotoxins produced by enterotoxigenic Escherichia and Salmonella are functionally, structurally, and evolutionarily related to cholera toxin. These toxins are also produced in the gut by colonizing bacteria. However, enterotoxins produced by some of the food-poisoning bacteria (Staphylococcus aureus, Clostridium perfringens, and Bacillus cereus) have quite different modes of action and are often ingested as preformed toxins produced from bacterial growth in contaminated food; growth of the pathogen in the host is unnecessary. C. perfringens enterotoxin is a cytotoxin and S. aureus enterotoxin is a superantigen (Table 28.4). Superantigens have a completely different mode of action, stimulating large numbers of immune lymphocytes and causing systemic as well as intestinal inflammatory responses. The toxin produced by Shigella dysenteriae, called Shiga toxin, and the Shiga-like toxin produced by enteropathogenic E. coli O157:H7 are protein synthesis-inhibiting AB toxins. The mode of action for this class of toxins is different, however, from that of the AB-type diphtheria toxin, as they specifically kill small intestine cells, leading to bloody diarrhea and related intestinal symptoms.

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