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Anaerobic Gram-Negative Bacilli
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>Bacteroides fragilis (most important clinically)
[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.
- 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
- 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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