Bacterial Toxins (Level III) PDF

BACTERIAL TOXINS for Level III  3] Cholera toxin Causative organism: Vibrio cholera It is a Gram-negative, comma-shaped bacterium cause cholera disease. It is a motile facultative anaerobic organism. Naturally live in brackish or saltwater and it is known to be food and water born...

BACTERIAL TOXINS for Level III  3] Cholera toxin Causative organism: Vibrio cholera It is a Gram-negative, comma-shaped bacterium cause cholera disease. It is a motile facultative anaerobic organism. Naturally live in brackish or saltwater and it is known to be food and water borne bacteria. Cholera disease: man usually gets infected from water or food which has been contaminated by feces of a cholera victim. The ingested bacterial cells multiply in the alimentary canal and after 2-5 days cause the sudden onset of nausea, vomiting, acute diarrhea and abdominal cramps. The loss of fluids from the gut in the form of liquid feces up to 20 liters/day leads to extreme dehydration resulting in a shock and death within few hours. Adequate therapy by intravenous replacement of fluids and electrolytes reduces mortality rate from 60% to 1%. The severe symptoms last for only few days and biopsy of intestines reveals superficial inflammation without invasion of the intestine.  Cholera Toxin structure (multi-domain toxin): The cholera toxin (named choleragen) is an enterotoxin protein toxin having mol. wt. 84000 Dalton. The toxin molecules is composed of 2 main fragments, Fragment "A" having mol. wt. 28000 daltons which upon treatment with thiols is converted into 2 smaller protein molecules named "A1" (23000 dalton) and "A2" (5000 daltons) fragments. The second main fragment "B" is 56000 daltons. This fragments is composed of an aggregate of 5 small protein units with mol. wt. 11000 daltons. Fragment "B" with its 5 sub-units are non toxic to host cells and its main function is specific binding to receptors at membrane surface.  The specific receptor is known to be one of the membrane lipids called "GM1" ganglioside. Mechanism of entry: - The 5 fragments of "B" sub-unit bind to "GM1" membrane ganglioside as their specific receptor. - The 5 fragments are inserted in the cell membrane creating a hydrophillic channel through which "A" subunit can diffuse into the host cell membrane. - Thiols present in host cell membranes reduce the disulphide bonds binding fragments "A1" and "A2" allowing the toxic "A1" fragment to diffuse into the cytoplasm to activate the adenylate cyclase membrane bound enzyme. Mode of action of Cholera toxin: - The toxin mainly acts on cells of ileal mucosa. - Fragment "A" is the toxic part of cholera specially sub-unit (A1) which is responsible for stimulating (activating) the adenylatecyclase enzyme (membrane bound protein complex)by binding to G-protein. - A1 fragment catalyses ADP – ribosylation of the target enzyme intracellularly in a manner similar to diphtheria toxin. - Continuous activity of adenylatecyclase enzyme by toxin action, causes reversing of the direction of chloride [Cl-] ions transport across the membranes of host cells. This results in active secretion of [Cl-] ions from the mucosal surface of intestines. - As a results, sodium [Na+] ions absorption across the membrane is reduced changing the ion balance causing passive efflux of water from mucosal surface of intestines in the form of severe diarrhea. 4] Enterotoxins of Escherichia coli (multi-domain toxin): Causative organism: Enterotoxigenic Escherichia coli (ETEC), Gram –ve non spore forming short rod bacterium. Its natural habitat is enteric cavity of man and animals. Non toxic strains are indicator organisms for sewage contamination of food and water. Enterotoxigenic strains of E. coli cause acute diarrhea in man and animals. The gene responsible for toxicity is located on transmissible plasmids in the bacterial cell. Enterotoxins produced by ETEC include heat-labile enterotoxin (LT) and heat-stable enterotoxin (ST). Infection with ETEC cause watery diarrhea with no blood nor leukocytes and abdominal cramping, fever, nausea with or without vomiting, chills, loss of appetite, headache and muscle aches. a. The heat labile enterotoxin (LT):resembles Cholera toxin - The (LT) toxin stimulates adenylatecyclase enzyme bound to the host cell membrane resembling the cholera toxin. - The (LT) toxin is synthesized as a high molecular weight polypeptide called the protoxin (mol. wt. is approximately 86,000 daltons). - "A1" subunit of LT toxin has mol. wt 25,500 daltons resembling cholera toxin subunit " A1" fragment, and five "B" subunits having mol. wt. 11,500 daltons (about the same size of cholera B- subunits). - The enterotoxic fragment "A1" of (LT) toxin results from proteolytic cleavage of the protoxin. - Antibodies prepared against cholera toxin (CT) cross-neutralize LT toxin activities. - Both LT and CT bind to ganglioside GM1 receptors on eukaryotic target cells of illeal mucosa via their B subunits and enter the host cells by a similar mechanism. - Mode of action of "A1" fragment is the same as that of cholera toxin, catalyzing the ADP ribosylation of adenylate cyclase enzyme in illeal mucosa cells, resulting in stimulation of adenylate cyclase activity causing secretion of fluid and electrolytes into the lumen and produces watery diarrhea. Thank You