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Shigella species

Background:

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Enterobacteriaceae  (Gram Negative Bacilli)  
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>Citrobacter species:
     1] Citrobacter koseri
     2] Citrobacter freundii
>Enterobacter species
     1] Enterobacter cloacae,
     2] Enterobacter aerogenes)
>Escherichia coli
>Klebsiella species:
     1] Klebsiella ozaenae
     2] Klebsiella pneumoniae
     3] Klebsiella rhinoscleromatis
>Morganella species 
     1] (Morganella morganii)
>Proteus species:
     1] Proteus mirabilis
     2] Proteus vulgaris
>Providencia species:
     1] Providencia_rettgeri
     2] Providencia stuartii
>Salmonella species:
     1] Salmonella enteritidis
     2] Salmonella typhi
     3] Other
>Serratia marcescens
>Shigella species:   led
     1] Shigella dysenteriae (serogroup A)
     2] Shigella flexneri
     3] Shigella boydii
     4] Shigella sonnei
     5] Other

Shigella:

  • Gram-negative, facultative anaerobic, nonspore forming, non-motile, rod-shaped bacteria closely related to Salmonella.
  • Causative agent of human shigellosis. During infection, it typically causes dysentery.
  • Shigella species are classified by four serogroups:
    • Serogroup A: S. dysenteriae (12 serotypes)
    • Serogroup B: S. flexneri (6 serotypes)
    • Serogroup C: S. boydii (18 serotypes)
    • Serogroup D: S. sonnei (1 serotype)
  • Groups A- C are physiologically similar; S. sonnei (group D) can be differentiated on the basis of biochemical metabolism assays. 
  • Three Shigella groups are the major disease-causing species:
    • S. flexneri is the most frequently isolated species worldwide, and accounts for 60% of cases in the developing world;
    • S. sonnei causes 77% of cases in the developed world, compared to only 15% of cases in the developing world; and
    • S. dysenteriae is usually the cause of epidemics of dysentery, particularly in confined populations such as refugee camps.
  • Shigella infection is typically via ingestion (fecal-oral contamination); depending on age and condition of the host, fewer than 100 bacterial cells can be enough to cause an infection.
  • Shigella causes dysentery that results in the destruction of the epithelial cells of the intestinal mucosa in the cecum and rectum.
  • Some strains produce the enterotoxin shiga toxin, which is similar to the verotoxin of E. coli O157:H7  and other verotoxin-producing Escherichia coli. Both shiga toxin and verotoxin are associated with causing hemolytic uremic syndrome.
  • The most common symptoms are diarrhea, fever, nausea, vomiting, stomach cramps and flatulence. The stool may contain blood, mucus or pus.

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Therapy:

Important considerations:  The choice of an agent should be based on local antimicrobial sensitivities, site of infection, cost, and comorbid conditions.   Generally, the most common agents/regimens are listed first.  Listed dosages may need to be adjusted for renal dysfunction.  

  1. Ciprofloxacin 500 mg orally q12h x 3 days
  2. Levofloxacin 500mg IV/PO once daily x 3 days
  3. Azithromycin 500mg orally once daily x 3 days
  4. Bactrim DS (TMP-SMX) 160/800mg po bid x 5 days
  5. Ceftriaxone 1-2 grams IV q24h x 5 days
Shigella species