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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.

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  1. Shigella Dr Sabrina Moyo Department of Microbiology and Immunology

  2. 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 x0.4 -0.6um rounded ends.

  3. 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

  4. Taxonomy Family Enterobacteriaceae • Shigella dysenteriae: most serious form of bacillary dysentery • Shigella flexneri: shigellosis in underdeveloped countries • Shigella sonnei: shigellosis in developed countries • Shigella boydii

  5. 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. boydii1-15 • group D - S. sonnei 1

  6. 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

  7. 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

  8. 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

  9. Epidemiology • Shigella occurs naturally in higher primates • Spread from human to human via the fecal-oral route •  Less frequently, transmission by ingestion of contaminated food or water •   Outbreaks usually occur in close communities; •   Secondary transmission occurs frequently

  10. Epidemiology •  Low infectious dose (102-104 CFU) with 1-3 day incubation period • Carriage of the organism persists for approximately one month following convalescence

  11. Pathogenesis & Immunity • Invasiveness involves attachment (adherence) and internalization • Controlled by a multi-gene virulence plasmid • Organisms penetrate through colonicmucosa • invade and multiply in the colonic epithelium • Not beyond the epithelium into the lamina propria

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. THANK YOU!

  18. Reference: www.Slideshare.com

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