Polymyxin B Does NOT Cover Anaerobes
Polymyxin B has no activity against anaerobic bacteria and should never be used as monotherapy when anaerobic coverage is needed. 1
Spectrum of Activity
Polymyxin B is specifically active against gram-negative aerobic bacilli only, with the following key characteristics 1:
- Active against: Almost all gram-negative aerobic bacilli (except Proteus species)
- NO activity against:
- All gram-positive bacteria
- All fungi
- Gram-negative cocci
- Anaerobic organisms
Clinical Implications for Polymyxin-Requiring Infections
When treating infections that require polymyxin B (typically multidrug-resistant gram-negative organisms like carbapenem-resistant Acinetobacter or Pseudomonas), you must add anaerobic coverage separately if the infection site involves anaerobes:
For Intra-Abdominal Infections
- Polymyxin B + metronidazole is required 2
- Alternative: Use tigecycline (which covers anaerobes) as part of combination therapy 2
For Hospital-Acquired/Ventilator-Associated Pneumonia
When polymyxin B is used for carbapenem-resistant pathogens:
- Intravenous polymyxin B (colistin or polymyxin B) is recommended 3
- Add inhaled colistin as adjunctive therapy 3
- No additional anaerobic coverage typically needed for HAP/VAP (anaerobes rarely cause these infections)
For Empyema
If polymyxin-requiring organisms are present:
- Must add metronidazole or clindamycin for anaerobic coverage 4
- Anaerobes frequently infect empyemas and should be covered empirically even with negative cultures 4
Critical Pitfall
Never assume polymyxin B provides broad-spectrum coverage. Its spectrum is extremely narrow—limited to aerobic gram-negative bacilli only. Any polymicrobial infection involving potential anaerobes (abdominal, pleural, bite wounds) requires separate anti-anaerobic agents like metronidazole, clindamycin, or a carbapenem with anaerobic activity 2, 4.