Shigella causes a dysentery called shigellosis, which is marked by crippling abdominal cramps and frequent defecation of watery stool filled with mucus and blood.
All shigellae have O Ag (polysaccharide) in their cell wall, and this antigen are used to divide the genus into 4 groups:.
All produce a similar disease that can vary in intensity. Sh.dysenteriae causes the severest form of dysentery.
Shigellae can produce an enterotoxins, invasion is the critical factor in pathogenesis. The evidence for this is that mutants that fail to produce enterotoxin but are invasive can still cause disease, whereas noninvasive mutants are nonpathogenic.
1. Shigellosis is only human disease. 3. Shigellae are the most effective pathogens among the enteric bacteria. Ingestion of as few as 10-100 microorganisms cause disease, whereas about 1 million Salmonella bacilli are required to produce symptoms. 6. Shigellosis is different from salmonellosis in that Shigella invades the villus cells of the large intestine, rather than the small intestine. In addition, it is not as invasive as Salmonella and does not perforate the intestine or invade the blood. It enters the intestinal mucosa. Once in the mucosa and causes extensive tissue destruction. As it multiplies, it gives off toxins. Toxins cause bloody diarrhea
Note the patches of blood and mucus, the erosion of the lining, and the absence of perforation.
As you can see from the bacterium’s name V.cholerae causes cholera. Cholera is the classic example of a very severe form of diarrhea. If cholera is left untreated, death can occur in less than 48 hours, and the mortality rate approaches 55%. Cholera is a human disease that originated in Asia but is now distributed worldwide. There have been 7 cholera pandemics since the 19 th century. The last most persistent pandemic, which began in1961 and continues until today, is due to a strain called the El Tor biotype. This pandemic began in Indonesia, spreading to South Asia, the Middle East, and parts of Europe and Africa.
Abilities of V.cholerae to grow in strong alkaline conditions and high salt concentrations allow the use of selective culture media that inhibit other bacteria but not V.cholerae. V.cholerae can live and multiply in soil and water (fresh and salt) in good condition – warm climate, presence of proteins
Showing its characteristic curved shape and single polar flagellum.
V.cholerae, the major pathogen in the vibrio genus, is the cause of cholera.
V.cholerae is divided into 2 groups according to the nature of its O cell wall antigen, agents of cholera belong to O1 serogroup and O139 serogroup. The O1 organisms have 2 biotypes, called cholerae classic and El Tor, and 3 serotypes, called Ogawa, Inaba and Hikojima. These features are used to characterize isolates in epidemiologic investigations.
Therefore organism can live in warm salt water it can contaminate marine shellfish, ingestion of these without adequate cooking can transmit the disease. 3. The pathogenesis of cholera is dependent on colonization of the small intestine by the organism and secretion of enterotoxin. For colonization to occur, large numbers of bacteria must be ingested, because the organism is particularly sensitive to stomach acid and the most quantity of vibrios perish in stomach. Persons with little or no stomach acid are much more susceptible to cholera.
Mechanisms of infectious diarrhea, (a) In a toxigenic infection, the microbe remains on the surface of epithelial cells and secretes toxin into the cells, (b) In an invasive infection, the microbe breaks down epithelial cells and forms ulcerations, loss of the intestinal lining, and bleeding Infectious diarrhea has two basic mechanisms. In the toxigenic type of disease, bacteria release enterotoxins that bind surface receptors of the small intestine. These toxins disrupt the physiology of epithelial cells and cause increased secretion of electrolytes and water loss, a condition called secretory diarrhea. The organism itself does not invade the tissues. Secretory diarrhea is characterized by its large volume, and there is little blood in the stool. This is the mechanism of cholera and some types of Escherichia coli infectious damage. In a more invasive diarrheal disease, the microbe invades the wall of the small or large intestine and disrupts its architecture, leading to gross injury. This form is attended by smaller fecal volume, pain in the rectum, blood in the stool, and ulceration of the inner mucosal lining. Salmonella, Shigella and enteroinvasive strains of E. coli are responsible for this type of intestinal disease.
During epidemics, clinical evidence is usually sufficient to diagnose cholera. But confirmation of the disease is often required for epidemiologic studies and detection of sporadic cases.
Properties of Shigellae <ul><li>Gram-negative rods </li></ul><ul><li>Nonmotile (do not have flagella) </li></ul><ul><li>Nonencapsulated </li></ul><ul><li>Not fastidious to laboratory medium </li></ul><ul><li>Do not ferment lactose (during first 48 hours) </li></ul><ul><li>Do not form H 2 S </li></ul><ul><li>They produce no gas from the fermentation of sugars </li></ul><ul><li>They are human parasites </li></ul><ul><li>Low infectious dose (< 100 bacteria) </li></ul><ul><li>Reproduce in the enterocytes of colon </li></ul>
Antigenic structure of Shigella <ul><li>O antigen. It used to divide the genus into 4 serogroup: A, B, C, D </li></ul><ul><li>K antigen. According it shigellae are divided into serotypes </li></ul>
Virulence factors of shigellae <ul><li>Adherence by pili </li></ul><ul><li>Invasion. Ipa-BCD antigen. Ensures entering of bacteria to the M-cells of large intestine, spreading to the underlying tissue, and apoptosis of phagocytes </li></ul><ul><li>Endotoxin that causes fever, general intoxication </li></ul><ul><li>Exotoxin ( Shiga toxin ) that has a number activities: </li></ul><ul><ul><li>Enterotoxic effect - inflammation of the underlying gut wall layer, degeneration of the villi, and local erosion that causes bleeding and heavy mucous secretion. </li></ul></ul><ul><ul><li>Cytotoxic effect - damage to the intestine, injury to the nerve cells, capillary endothelium, myocardial and kidney cells. </li></ul></ul>
Pathogenesis of shigellosis <ul><li>Reservoir of shigellae are ill humans only (not animals) </li></ul><ul><li>Mechanism of transmission is fecal-oral . The 4 F’s – fingers, flies, food, and feces – are the principal factors in transmission. </li></ul><ul><li>Infectious dose is small and depend on virulence of Shigella, for Sh.disenteriae ID is 10 cells, for Sh.sonnei – at least 10 4 cells. </li></ul><ul><li>Incubation period is 1 to 4 days. </li></ul><ul><li>Sh.dysenteriae (the most virulent among shigellae) causes the most severe disease, Sh.sonnei (the least virulent among shigellae) causes mild disease. </li></ul><ul><li>Manifestations of dysentery are fever, vomiting, frequent liquid stools containing blood and pus, abdominal cramps, and intestinal pain but unproductive urge to defecate. Less common symptoms are headache, stiff neck, convulsions. </li></ul>
Shigellosis <ul><li>within 2-3 days </li></ul><ul><ul><li>epithelial cell damage </li></ul></ul>Destructive effect of shigellae resembles effect of enteroinvasive E. coli
The appearance of large intestinal mucosa in Shigella (bacillary) dysentery
Diagnosis of bacillary dysentery <ul><li>Bacteriologic method – isolation of pure culture from feces and identification: </li></ul><ul><ul><ul><li>Morphology properties, Gram-staining </li></ul></ul></ul><ul><ul><ul><li>Biochemical properties </li></ul></ul></ul><ul><ul><ul><li>Antigenic properties (serological identification) </li></ul></ul></ul><ul><li>Gen diagnosis - PCR </li></ul><ul><li>Serological method - revealing of antibody titer increasing in pair serum or specific IgM exposure </li></ul><ul><li>Bacterioscopy method can not be used </li></ul>
Treatment <ul><li>Fluid and electrolyte replacement . </li></ul><ul><li>In severe cases antibiotics are indicated, in mild cases, non antibiotics are used. Due to multiple drug resistance (connected with plasmids) antibiotics sensitivity tests must be performed. </li></ul><ul><li>Antiperistaltic drugs are contraindicated in shigellosis, because they prolong the fever, diarrhea, excretion of the microorganisms, and can provoke ulcers formation. </li></ul>
Properties of Vibrio cholerae <ul><li>Gram negative motile rod </li></ul><ul><li>comma shaped </li></ul><ul><li>facultative anaerobe </li></ul><ul><li>oxidase positive </li></ul><ul><li>it has one flagellum at one end </li></ul><ul><li>it is able to tolerate strong alkaline conditions and high salt concentrations </li></ul><ul><li>readily cultivated </li></ul>
Vibrio cholerae. Micrograph of bacteria in tissue Curved shape and single polar flagellum
Genuses of Vibrionaceae that are medically important Vibrionaceae Vibrio Aeromonas Plesiomonas <ul><li>V. cholerae causes cholera </li></ul><ul><li>V. parahaemolyticus is primarily marine organism (halophilic). It causes diarrhea associated with eating raw or improperly cooked seafood </li></ul><ul><li>V. vulnificus is found in seawater (halophilic). It causes severe skin and soft tissue infections, can cause a rapidly fatal septicemia in immunocompromised people </li></ul>
Antigenic structure of vibrios <ul><li>O (somatic) antigen. There are 150 serogroup of vibrios </li></ul><ul><li>H (flagella) antigen. It is common antigen for all vibrios </li></ul>
Classification of Vibrio cholerae Ogawa , Inaba , Hikojima Serotypes Classic and El Tor Biotypes Agents of cholera belong to O1 serogroup and O139 serogroup ( a new strain of V.cholerae that appeared in India in late 1992 ) Serogroup cholerae Species Вид Vibrio Genus Vibrionaceae Family
Chemogroups of vibrios according to Heiberg classification (1935) V. cholerae belongs to I chemogroup - - - VI - - + V + + - IV + + + ІІІ + - - ІІ + - + І Sucrose Arabinose Mannose Fermentation of carbohydrates Chemogroup
Differentiation of biotypes V. cholera classic and El Tor - + Sensitivity to polymyxin + - Susceptibility to group II (El Tor) bacteriophage - + Susceptibility to group IV (C) bacteriophage + - Hemolysis + - Agglutination of chicken erythrocytes V. cholerae El Tor V. cholerae classic Feature
V. cholerae classic and El Tor growth on the blood agar Classic El Tor
Virulence factors of V.cholerae <ul><li>Adherence to mucosa layer of small intestine by pili </li></ul><ul><li>Enzyme of virulence – mucinase, hyaluronidase. </li></ul><ul><li>Exotoxin (cholerogen). The heat labile enterotoxin that consist of an A (active) and a B (binding) subunits. The B subunit binds to a ganglioside receptor on the surface of the enterosyte. The A subunit is inserted into the cytosol, where cause activation of the enzyme adenylate cyclase. The resulting overproduction of cAMP (cyclic adenosine monophosphate) stimulates secretion of chloride ions and water, leading to a massive watery diarrhea without inflammatory cells. </li></ul><ul><li>Endotoxin cause inhibition of phagocytosis, decrease of blood pressure, general intoxication </li></ul>
Pathogenesis of cholera <ul><li>Sours of infection are ill humans of carries. </li></ul><ul><li>The main animal reservoirs are marine shellfish, such as shrimp and oysters. </li></ul><ul><li>Transmission fecal-oral, with feces contaminated water or food. </li></ul><ul><li>Infectious dose is approximately 1 billion of bacteria. </li></ul><ul><li>Incubation period is a few hours to a few days. </li></ul>
Pathogenesis of cholera <ul><li>Colonization of small intestine. Vibrio penetrate the mucous barrier of duodenum using their flagellum, adhere to the microvilli of the epithelial cells and multiply there. </li></ul><ul><li>Adherence is related to secretion of the bacterial enzyme mucinase , which dissolves the protective glycoprotein coating over the intestinal cells. </li></ul><ul><li>Visible damage intestinal epithelial cells is absence. </li></ul>Vibrios Villus surface
Cholera - attachment V. cholerae bacteria, unlike the shigellae, do not penetrate into the intestinal cells
Mechanism of infectious diarrhea Intestinal epithelium Secretion of fluids Bacteria Non-blooding diarrhea Blooding diarrhea
Pathogenesis of cholera. Symptoms of cholera <ul><li>Due to cholerogen symptoms of disease occur. Vomiting, secretory diarrhea, dehydration lead to muscle, circulatory, and neurological symptoms and death. </li></ul><ul><li>Watery diarrheal fluid contains flecks of mucus and therefore has been described as “rice-watery stool”. </li></ul><ul><li>Fluid losses of nearly one liter per hour ( to 20 liters a day ) have been reported in severe cases, and an untreated patient can lose up to 50% of body weight during the course of the disease. </li></ul><ul><li>The diarrhea causes loss of blood volume, acidosis from bicarbonate loss, and potassium depletion that predispose the patient to muscle cramps, and, in young children, coma and convulsions. </li></ul><ul><li>The loss of fluid and electrolytes leads to cardiac and renal failure. </li></ul><ul><li>Secondary circulatory consequences can include hypotension, tachycardia, cyanosis, and collapse from shock within 18 to 24 hours. </li></ul><ul><li>Mortality rate from cholera approaches 55%. </li></ul>
Diagnosis <ul><li>Material from patient – stool samples </li></ul><ul><li>Methods of diagnosis: </li></ul><ul><li>Bacterioscopy. </li></ul><ul><ul><li>Direct dark-field microscopic observation. Curved cells with darting motility. </li></ul></ul><ul><ul><li>Gram stained swab of feces. Gram negative curved rods </li></ul></ul><ul><li>Indication of specific antigen of the V. cholerae with immunofluorescence test. </li></ul><ul><li>Bacteriological method. Isolation and identification of pure culture. </li></ul><ul><li>Serological method . Detection of rising antitoxin titer in the serum </li></ul>
Therapy <ul><li>Replacement of water and electrolytes </li></ul><ul><li>Antimicrobial therapy – antibiotics </li></ul>
Prevention <ul><li>Vaccines are available, but </li></ul><ul><ul><li>partially effective </li></ul></ul><ul><ul><li>not generally used </li></ul></ul><ul><ul><li>used only for international travelers and people, living in endemic regions </li></ul></ul><ul><li>Types of vaccines: </li></ul><ul><li>Killed cholera vibrios. But it protect for only 6 months or less. </li></ul><ul><li>Oral attenuated vaccine containing live vibrios. It is more effective. </li></ul>
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