Shigella dysentery bacterium 1 General Characteristics of Shigella

Shigella (dysentery bacterium ) 1

General Characteristics of Shigella ØColiform bacilli (enteric rods) Ø Nonmotile gram-negative facultative anaerobes Ø Short rods Ø Non –encapsulated Ø non-sporeforming 2

Classification The genus is divided into four serogroups with multiple serotypes • Shigella dysenteriae (12 serotypes) • Shigella flexneri(6 serotypes) • Shigella sonnei (1 serotype) • Shigella boydii (18 serotypes) 3

Classification • Based on Mannitol fermentation: A. Shigella dysenteriae – Mannitol non fermented B. Shigella flexneri C. Shigella sonnei Mannitol fermenters D. Shigella boydii 4

Classification • 4 well-defined species – S. dysenteriae-produces a more serious disease than the other species – S. flexneri-most commonly isolated in developing countries – S. boydii – S. sonnei-cause a mild form of diarrhea, patients are ambulatory 5

Culture characteristics • facultatively anaerobic • PH 6. 4 to 7. 8 • Temp 10ºC-40ºC (optimum temp 37 C) • Colony characters: • Nutrient agar: circular, convex , colorless , smooth surface. • Mac. Conkey agar: Non-Lactose fermenting, colourless colonies except S. sonnei which is late lactose fermenter. 6

Selective media: • XLD: colony appear pinkish to reddish due to xylose fermentation. • HEA(Heaktoen Enteric Agar ): It appear green to blue green colonies • Salmonella- Shigella agar 7

Biochemical Characteristics • All ferment glucose, no gas. • Lactose not fermented, except S. sonnei(LLF). • Ferment mannitol except Shigella dysenteriae • Catalase +ve except Shigella dysenteriae type 1 • Urease test(-) • Citrate(-) • Gelatin liquefaction(-) • TSI: K/A except S. sonnei, A/A 8

Epidemiology • Shigellosis is endemic in developing countries were sanitation is poor • Typically 10 to 20 percent of enteric disease • 50% of the bloody diarrhea or dysentery of young children, can be characterized as shigellosis 9

Epidemiology and Clinical Syndromes of Shigella Shigellosis = Generic term for disease Ø Ø Ø Ø Low infectious dose (10 -100 CFU) Humans are only reservoir Transmission by fecal-oral route Incubation period : 1 -3 days Watery diarrhea with fever; changing to dysentery Outbreaks in day care centers, nurseries, institutions Leading cause of infant diarrhea and mortality in developing countries 10

Pathogenesis and Virulence Factors (cont. ) Virulence attributable to: Ø Invasiveness · Attachment and internalization · virulence plasmid regulated by multiple chromosomal genes Ø Exotoxin (Shiga toxin) Ø Intracellular survival & multiplication 11

Pathogenesis and Virulence Factors (cont. ) Invasiveness in Shigella-Associated Dysentery Ø Penetrate through mucosal surface of colon and invade and multiply in the colonic epithelium but do not typically invade beyond the epithelium into the lamina propria. Ø Preferentially attach to and invade into M cells in Peyer’s patches (lymphoid tissue, i. e. , lymphatic system) of small intestine 12

Pathogenesis and Virulence Factors (cont. ) Invasiveness in Shigella-Associated Dysentery(cont. ) Ø M cells typically transport foreign antigens from the intestine to underlying macrophages, but Shigella can lyse the phagocytic vacuole (phagosome) and replicate in the cytoplasm Ø Actin filaments propel the bacteria through the cytoplasm and into adjacent epithelial cells with cell -to-cell passage, thereby effectively avoiding antibody-mediated humoral immunity 13

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Pathogenesis and Virulence Factors (cont. ) Characteristics of Shiga Toxin Ø Enterotoxic, neurotoxic and cytotoxic Ø Encoded by chromosomal genes Ø Two domain (A-5 B) structure Ø Similar to the Shiga-like toxin of enterohemorrhagic E. coli (EHEC) 15

Shiga Toxin Effects in Shigellosis Cytotoxic Effect: Ø B subunit of Shiga toxin binds host cell glycolipid Ø A domain is internalized via receptor-mediated endocytosis Ø Causes irreversible inactivation of the 60 S ribosomal subunit, thereby causing: · Inhibition of protein synthesis · Cell death · Microvasculature damage to the intestine · Hemorrhage (blood & fecal leukocytes in stool) Neurotoxic Effect: Fever, abdominal cramping are considered signs of neurotoxicity 16

Shiga Toxin Effects in Shigellosis Enterotoxic Effect: Ø Adheres to small intestine receptors Ø Blocks absorption (uptake) of electrolytes, glucose, and amino acids from the intestinal lumen 17

Infection cycle of Shigella Organisms adhere to colonic lymphoid follicles STOMACH Undergo parasite directed phagocytosis Resist intracellular killing SMALL GUT Move directly from cell to cell Shiga toxin exacerbate inflammation by causing ischemia & hemorrhage Ulcers Rarely lead to bacteremia LARGE GUT 18

Clinical Manifestations • Symptoms of shigellosis include abdominal pain, watery diarrhea, and/or dysentery (multiple scanty, bloody, mucoid stools). Other signs may include abdominal tenderness, fever, vomiting, dehydration, and convulsions 19

Pathogenesis of Shigella Shigellosis Two-stage disease: Ø Early stage: · Watery diarrhea attributed to the enterotoxic activity of Shiga toxin following ingestion and noninvasive colonization, multiplication, and production of enterotoxin in the small intestine · Fever attributed to neurotoxic activity of toxin Ø Second stage: · Adherence to and tissue invasion of large intestine with typical symptoms of dysentery · Cytotoxic activity of Shiga toxin increases severity 20

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Complications 1. Hemolytic Uremic Syndrome (HUS) 2. Intussusception 3. Reitz syndrome: Arthritis, Conjunctivitis, Urethretis 23

Lab diagnosis • Specimen: Stool with blood and mucus • Microscopy: Plenty of pus cells, RBC’s in roulaex formation. • Culture: Nonselective and selective media • Colonies identified by biochemical reactions and specific antisera. 24