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Shigella Dr Sabrina Moyo Department of Microbiology and Immunology Definition • • • • An enterobacteriaceae Gram negative bacilli. Readily growth O2 + An O2. Metabolically active, fermenting a variety of substrates. • Mostly non-motile, non sporing, non acid fast, 2-4um x 0.4 -0.6um rounded ends. Morphology & Physiology • Small Gram-negative, facultatively anaerobic, coliform bacillus • Non-motile (no H antigen) • Possess capsule (K antigen) and O antigen • K antigen not useful in serologic typing, but can interfere with O antigen determination • O antigens: A, B, C, D correspond respectively to the four species • Non-lactose fermenting • Bile salts resistant: trait useful for selective media ferment glucose reduce nitrates (NO3 to NO2 or N2) are oxidase negative Taxonomy Family Enterobacteriaceae 1. Shigella dysenteriae: most serious form of bacillary dysentery 2. Shigella flexneri: shigellosis in underdeveloped countries 3. Shigella sonnei: shigellosis in developed countries 4. Shigella boydii Taxonomy • Classification: 4 groups or species on the based on difference in O antigen and some biochemical reactions • Group A - S. dysenteriae 1-10 • group B - S. flexneri 1-6 • group C - S. boydii 1-15 • group D - S. sonnei 1 Clinical Syndromes (Shigellosis) • Ranges from asymptomatic infection to severe bacillary dysentery • Two-stage disease: watery diarrhea changing to dysentery with frequent small stools with blood and mucus, tenesmus, cramps, fever Early stage: • Watery diarrhea attributed to the enterotoxic activity of Shiga toxin • Fever attributed to neurotoxic activity of toxin Clinical Syndromes Process involves: 1. Ingestion 2. Non-invasive colonization and cell multiplication • 3. Production of the enterotoxin by the pathogenic bacteria in the small intestine; Second stage: • Adherence to and tissue invasion of large intestine • Typical symptoms of dysentery • Cytotoxic activity of Shiga toxin increases severity Epidemiology • Shigellosis is a major cause of diarrheal disease (developing nations) • Major cause of bacillary dysentery (severe second stage form of shigellosis) • Leading cause of infant diarrhea and mortality (death) in developing countries Epidemiology • Shigella occurs naturally in higher primates • Spread from human to human via the fecaloral route • Less frequently, transmission by ingestion of contaminated food or water • Outbreaks usually occur in close communities; • Secondary transmission occurs frequently Epidemiology • Low infectious dose (102-104 CFU) with 1-3 day incubation period • Carriage of the organism persists for approximately one month following convalescence Pathogenesis & Immunity • Invasiveness involves attachment (adherence) and internalization • Controlled by a multi-gene virulence plasmid • Organisms penetrate through colonic mucosa • invade and multiply in the colonic epithelium • Not beyond the epithelium into the lamina propria Pathogenesis & Immunity • Bacterial cells preferentially attach to and invade into M cells in Peyer's patches of small intestine • M cells typically transport foreign antigens from the intestine to underlying macrophages, • Shigella can lyse the phagocytic vacuole (phagosome) and replicate in the cytoplasm Pathogenesis & Immunity • Exotoxin (Shiga toxin) is neurotoxic, cytotoxic, and enterotoxic, encoded by chromosomal genes, • Enterotoxic effect: Shiga toxin adheres to small intestine receptors • Blocks absorption (uptake) of electrolytes, glucose, and amino acids from the intestinal lumen Pathogenesis & Immunity • Cytotoxic effect: B subunit of Shiga toxin binds host cell glycolipid in large intestine, • Inactivate the 60S ribosomal subunit, • Inhibit protein synthesis, causing cell death, microvasculature damage to the intestine, and hemorrhage (blood and fecal leukocytes in stool) • Neurotoxic effect: Fever, abdominal cramping are considered signs of neurotoxicity Laboratory Identification: • Closely related to Escherichia • Species (serogrouping and biochemical analysis • Stool specimens and rectal swabs should be cultured soon after collection or placed in appropriate transport medium (Cary-Blair medium) • Readily isolated on selective/differential agar media (XLD, SS, and brilliant green agar • Lactose nonfermenter Treatment, Prevention & Control: • Dehydration is problem to attend • Treat carriers, major source of organisms; Cirpflxacin , Erythromycin • Antibiotic resistance is a major problem • Proper sewage disposal and water chlorination • Oral vaccines of Shigella: E. coli hybrids or Shigella mutants offers immunity for six months to one year • THANK YOU! Reference: www.Slideshare.com