Bacteroides fragilis


Anaerobic Gram-Negative Bacilli
>Bacteroides fragilis (most important clinically)led
    [Other Bacteroides species  – less common clinically:  (B. acidifaciens, B. gracilis,
         B. oris, B. ovatus,  B. pyogenes , B. stercoris, B. vulgatus)]
>Bacteroides melaninogenicus (reclassified and split into Prevotella melaninogenica
     and Prevotella intermedia.)
>Fusobacterium necrophorum
>Porphyromonas gingivalis

Bacteroides fragilis:

  • Gram-negative bacillus bacterium species, and an obligate anaerobe of the gut.
  • B. fragilis group is the most commonly isolated Bacteroidaceae in anaerobic infections (especially those that originate from the gastrointestinal flora).
  • B. fragilis is the most prevalent organism in the B. fragilis group, accounting for 41% to 78% of the isolates of the group.
  • These organisms are resistant to penicillin by virtue of production of beta-lactamase, and by other unknown factors.
  • B. fragilis is involved in 90% of anaerobic peritoneal infections.

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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.

  1. Mild to moderate infections:
    • Metronidazole 500mg IV/PO every 6 hours
    • Augmentin 875/125 mg orally twice daily  OR Ampicillin-sulbactam (Unasyn®) 1.5 – 3.0 grams IV q6h
    • Cefoxitin 1-2 gm IV q6h (increasing resistance)
    • Moxifloxacin 400mg orally/IV qd  (increasing resistance)
    • Tigecycline 100 mg IV x 1, then 50 mg q12h
    • Combination examples:  [ceftriaxone, cefotaxime, ciprofloxacin (IV/Oral), or levofloxacin (IV/Oral)]  PLUS  Metronidazole 500mg IV/PO every 6 hours
  2. Severe infections:
    • Piperacillin-tazobactam (Zosyn ®)  3.375 to 4.5  grams IV q6h
    • Imipenem 500mg IV every 6 hours [Range: 250-1000 mg q6-8h]
    • Meropenem 0.5 – 1 gram IV q8h 
    • Doripenem 500 mg IV q8h
    • Combination examples:  [Cefepime, ceftazidime, ciprofloxacin (IV), or levofloxacin (IV) ]  PLUS  Metronidazole 500mg IV every 6 hours


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