USMLE

Shigella

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Bacteria - Gram Negative
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Shigella 

  • Includes S. sonnei, S. dysenteriae, S. boydii, and S. flexneri
  • Characteristics
    • Salmonella and Shigella spp. are all gram-negative rods, non-lactose fermenters, oxidase negative, and invade the GI tract via M cells of Peyer patches
    • Gram-negative rod
      • First test in algorithm, narrows scope of bacteria
    • Does not ferment lactose
      • Second test in algorithm
    • Oxidase negative
      • Third test in algorithm
    • Does not produce H2S 
      • When grown on triple sugar iron agar
      • Fourth test in algorithm, narrows to Shigella spp.
      • Contrast w Salmonella spp. - H2S positive 
    • Non-motile (no flagella)
      • Contrast w Salmonella spp. - motile 
  • Virulence factors
    • Endotoxin
    • Shiga toxin (enterotoxin)
      • Inactivates 60S ribosomal subunit
        • inhibits protein synthesis in intestinal cells
        • E. coli’s shiga-like toxin (verotoxin) has the same structure, mechanism 
      • Toxin functionally plays a minor role in pathogenicity
  • Transmission
    • Reservoir is humans
      • Fecal-oral (contaminated food or water) 
        • Especially undercooked meat 
        • More common in children 
    • Infectious dose (ID50) is low
      • Acid-stable (resistant to gastric acid)
        • requires only a small inoculum to invade and survive
      • Contrast w Salmonella spp (acid labile, requires high infectious dose)
  • Pathogenesis
    • Invades GI mucosa via M cells of Peyer patches
      • M (microfold) cells make up lymphatic tissue that samples intestinal mucosa for antigens
      • Escapes phagosomes and spreads cell-to-cell in mucosa
        • no hematogenous spread (Contrast w Salmonella spp.)
      • Causes ulceration and bleeding
      • Direct bacterial spread is main cause of pathogenicity
        • Not toxin-mediated damage (despite production of Shiga toxin)
  • Presentation:  Shigellosis
    • GI manifestations 
      • Fever
      • Crampy abdominal pain
      • Bloody mucoid stools (bacillary dysentery)
        • Contrast w Salmonella typhi - non-bloody diarrhea
  • Treatment 
    • Supportive care (rehydration)
    • Antibiotics in severe cases
      • Ciprofloxacin, ceftriaxone, or TMP-SMX
    • Zinc in young children shortens duration of disease and improves weight gain