Lect#21.pptx

Lecture #21Vibrio cholerae Lecture Overview Cholera is caused by the bacterium Vibrio cholerae. It is one of the most devastating waterborne diseases. We will look at the various strains of cholera, patterns of epidemiology as well as the cholera toxin. We will also look at treatments of cholera....

Lecture #21Vibrio cholerae Lecture Overview Cholera is caused by the bacterium Vibrio cholerae. It is one of the most devastating waterborne diseases. We will look at the various strains of cholera, patterns of epidemiology as well as the cholera toxin. We will also look at treatments of cholera. Lecture Objectives Cholera toxin is a model for the actions of bacterial toxins in pathogenesis. After this lecture and reading the text you should be able to describe the mode of action of cholera toxin and diptheria toxin. You should be familiar with what an A-B toxin is and ADP ribosylation by a toxin. We will also look at the an A-B toxin produced by Corynebacterium diphtheriae. Microorganisms and Concepts Vibrio cholerae • reservoir of Vibrio • epidemiology • Cholera toxin (CTX) • Corynebacterium diphtheriae : Diptheria toxin Alfred Rethel: First outbreak of cholera in Paris during a masquerade, 1851. -The genus Vibrio consists of Gram-negative straight or curved rods, motile by means of a single polar flagellum. -Vibrios are capable of both respiratory and fermentative metabolism. -In most ways vibrios are related to enteric bacteria - Vibrios are distinguished from enterics by being oxidase-positive and motile by means of polar flagella. -Of the vibrios that are clinically significant to humans, Vibrio cholerae, the agent of cholera, is the most important. -V. cholerae and V. parahaemolyticus are pathogens of humans. Both produce diarrhea, but in ways that are entirely different. - V. parahaemolyticus is an invasive organism affecting primarily the colon; V. cholerae is noninvasive, affecting the small intestine through secretion of an enterotoxin. Transmission - contaminated water and food. - Vibrio cholerae is often found in the aquatic environment and is part of the normal flora of brackish water and estuaries. - It is often associated with algal blooms (plankton), which are influenced by the temperature of the water. Human beings are also one of the reservoirs of the pathogenic form of Vibrio cholerae. - After a disaster, this is a very real danger, since regular, clean water and food supplies are often unavailable. The Place Cholera Calls Home Understanding how cholera lives and moves through water has been a goal. But where do those bacteria come from before they enter the rivers and wells? - connection between cholera and the marine environment. -plankton proved most interesting. Classifying these microbes, the researchers found the expected groups of zooplankton (small animals, such as copepods) and phytoplankton (small plants, such as green algae). Left: Sampling water for cholera. Right: A microscopic view of a female, potentially cholera-carrying copepod. Symptoms -V. cholerae produces cholera toxin, the model for enterotoxins, whose action on the mucosal epithelium is responsible for the characteristic diarrhea of the disease cholera. -In its extreme manifestation, cholera is one of the most rapidly fatal illnesses known. A healthy person may become hypotensive within an hour of the onset of symptoms and may die within 2-3 hours if no treatment is provided. -More commonly, the disease progresses from the first liquid stool to shock in 4-12 hours, with death following in 18 hours to several days. Epidemiology: a branch of medical science that deals with the incidence, distribution, and control of disease in a population History and spread of epidemic cholera -Cholera has smoldered in an endemic fashion on the Indian subcontinent for centuries. -There are references to deaths due to dehydrating diarrhea dating back to Hippocrates and Sanskrit writings. - In 1883, Robert Koch successfully isolated the cholera vibrio from the intestinal discharges of cholera patients and proved conclusively that it was the agent of the disease. - the vibrio responsible for the seventh pandemic, now in progress, is known as V. cholerae O1, biotype El Tor. The current seventh pandemic began in 1961 when the vibrio first appeared as a cause of epidemic cholera in Celebes (Sulawesi), Indonesia. The disease then spread rapidly to other countries of eastern Asia and reached Bangladesh in 1963, India in 1964, and the USSR, Iran and Iraq in 1965-1966. -In 1970 cholera invaded West Africa - In 1991 cholera struck Latin America, where it had also been absent for more than a century. Within the year it spread to 11 countries, and subsequently throughout the continent. - Until 1992, only V. cholerae serogroup O1 caused epidemic cholera New previously unrecognized serogroup of V. cholerae, designated O139. Antigenic Variation and LPS Structure in Vibrio cholerae - flagellar antigens of V. cholerae are shared with many water vibrios and therefore are of no use in distinguishing strains causing epidemic cholera. -O antigens, however, do distinguish strains of V. cholerae into 139 known serotypes. Almost all of these strains of V. cholerae are nonvirulent. -Until the emergence of the Bengal strain (which is "non-O1") a single serotype, designated O1, has been responsible for epidemic cholera. -The first long-distance spread of cholera to Europe and the Americas began in 1817 and by the early 20th century, six waves of cholera had spread across the world in devastating epidemic fashion. -Since then, until the 1960s, the disease contracted, remaining present only in southern Asia. In 1961, the "El Tor" biotype (distinguished from classic biotypes by the production of hemolysins) reemerged to produce a major epidemic in the Philippines and to initiate a seventh global pandemic. The global spread of cholera during the seventh pandemic, 1961-1971 Locations of water pumps and cholera deaths from John Snow's map (the Broad Street pump is the blue symbol at the center of the map). Cholera outbreak Soho England, 1854. John Snow; father of modern epidemiology. The Toilet Bed: “Cholera Cot” Rice Water Stool Colonization of the Small Intestine There are several characteristics of pathogenic V. cholerae that are important determinants of the colonization process. These include adhesins, neuraminidase, motility, chemotaxis and toxin production. Adhesin-cell surface molecules that mediate attachment between pathogen and host. Neuraminidase-break glycosidic bonds of glycoproteins and glycolipids of connective tissue. Vibrio cholerae attachment and colonization in experimental rabbits. Cholera Toxin -Cholera toxin(CTX) activates the adenylate cyclase enzyme in cells of the intestinal mucosa leading to increased levels of intracellular cAMP. This activates a salt channel called CFTR. In turn there is massive secretion of H20, Na+, K+, Cl-, and HCO3- into the lumen of the small intestine. -The effect is dependent on a specific receptor, monosialosyl ganglioside (GM1 ganglioside) present on the surface of intestinal mucosal cells. -The bacterium produces an invasin, neuraminidase, during the colonization stage which has the interesting property of degrading gangliosides to the monosialosyl form, which is the specific receptor for the toxin. Thus, the net effect of the toxin is to cause cAMP to be produced at an abnormally high rate which stimulates mucosal cells to pump large amounts of Cl- into the intestinal contents. -H2O, Na+ and other electrolytes follow due to the osmotic and electrical gradients caused by the loss of Cl-. -The lost H2O and electrolytes in mucosal cells are replaced from the blood. Thus, the toxin-damaged cells become pumps for water and electrolytes causing the diarrhea, loss of electrolytes, and dehydration that are characteristic of cholera. cystic fibrosis transmembrane conductance regulator Cholera toxin is released from bacteria in the gut lumen and binds via the B subunit to GM1 receptors on enterocytes, triggering endocytosis. Following activation in the cytosol of an infected cell, the A subunit enzymatically activates a G protein and locks it into its GTP-bound form through an ADP-ribosylation reaction. Constitutive G protein activity leads to activation of adenylyl cyclase and increased cAMP levels. The ADP-ribosylation causes the Gαs subunit to lose its catalytic activity in hydrolyzing GTP to GDP + Pi so it remains activated longer than normal Toxins Subvert Host Function  AB Toxins B subunit binds to host cell   A Delivers A subunit to cell Often 5 B subunits form a pore for A entry subunit has toxic activity  ADP-Ribosyltransferase   Diphtheria toxin Cholera toxin Vibrio cholerae http://www.youtube.com/watch?v=5yGbxyOpZRk ADP-Ribosylating Toxins  Cholera toxin  Ribosylates to overactivate adenylate cyclase  cAMP activates ion transport, water follows  Uncontrollable diarrhea  Diphtheria toxin  Ribosylates elongation factor 2  Blocks ribosome function, cell dies  Corynebacterium diphtheriae Forms pseudomembrane over trachea Diphtheria • caused by Corynebacterium diphtheriae – gram-positive – lysogenized strains produce an exotoxin that inhibits protein synthesis and is responsible for pathogenesis – resistant to drying • airborne transmission by nasopharyngeal secretions – crowding increases likelihood of transmission 33 34 -Diptheria toxin: ADP ribosylation of EF2 Diphtheria… • diagnosis – observation of pseudomembrane in throat – bacterial culture • treatment, prevention, and control – antitoxin given to neutralized unabsorbed exotoxin in patient’s tissues – antibiotic therapy – active immunization with DPT (diphtheriapertussis-tetanus) or acellular DTap vaccine 36 How Cholera Became a Killer -Most strains of the bacterial species Vibrio cholerae are harmless. But one strain is lethal. -What caused this deadly transformation? A phage (= bacterial virus), according to microbiologist Matthew Waldor. -Waldor, and co-workers discovered the virus while studying the stretch of bacterial DNA known to include the gene, called CTX, that codes for the cholera toxin. -They suspected that a virus might have infected the bacteria with the gene, since viruses often insert their own genetic material into bacteria. -Another possibility was that different strains of bacteria were swapping genes, a routine occurrence among wild strains. Role of lytic phages in chitin-mediated uptake of foreign DNA by V. cholerae in the aquatic environment. Lytic phages acting on V. cholerae cells release DNA from the donor cells. The free DNA fragments are taken up by recipient strains, aided by chitin-induced competence. Possible recombination between the foreign DNA and the host chromosome can cause genetic changes in the recipient cell. What should travelers do to avoid getting cholera? The risk for cholera is very low for Canadian. travelers visiting areas with epidemic cholera. All travelers to areas where cholera has occured should observe the following recomendations: - Drink only water that you have boiled or treated with chlorine or iodine. - Eat only foods that have been thoroughly cooked and are still hot, or fruit that you have peeled yourself. -Avoid undercooked or raw fish or shellfish, including ceviche. -Make sure all vegetables are cooked -avoid salads. -Avoid foods and beverages from street vendors. A simple rule of thumb is "Boil it, cook it, peel it, or forget it. " http://www.youtube.com/watch?v=jG1VNSCsP5Q Can it be treated? A mixture of sugar and certain salts that the body needs must be mixed with clean water and drunk in large amounts