Polymyxin B Spectrum of Coverage
Polymyxin B has bactericidal activity against almost all gram-negative bacilli, with the notable exception of the Proteus group, Serratia species, and Burkholderia cepacia. 1
Primary Coverage
Polymyxin B demonstrates excellent activity against multidrug-resistant (MDR) gram-negative bacteria, specifically:
Highly Susceptible Organisms (MIC90 ≤2 mg/L):
- Pseudomonas aeruginosa - including carbapenem-resistant and MDR strains 2, 3
- Acinetobacter baumannii - including carbapenem-resistant strains 4, 3
- Klebsiella pneumoniae - including carbapenem-resistant and ESBL-producing strains 2, 3
- Escherichia coli (>98% susceptible) 3
- Citrobacter species (>98% susceptible) 3
Moderately Susceptible Organisms:
- Enterobacter species (83.3% susceptible) 3
- Stenotrophomonas maltophilia (72.4% susceptible) 3
- Haemophilus influenzae - specifically for meningeal infections 1
- Klebsiella pneumoniae (Aerobacter aerogenes) - specifically for bacteremia 1
Organisms NOT Covered
All gram-positive bacteria are intrinsically resistant to polymyxin B 1. This includes:
- Staphylococcus aureus (including MRSA)
- Streptococcus species
- Enterococcus species
Resistant Gram-Negative Organisms:
- Proteus mirabilis (0.7% susceptible) 3
- Indole-positive Proteus species (1.3% susceptible) 3
- Serratia species (5.4% susceptible) 3
- Burkholderia cepacia (11.8% susceptible) 3
- All fungi 1
- Gram-negative cocci 1
Clinical Context
Polymyxin B is reserved as a last-resort agent for serious infections caused by carbapenem-resistant or extensively drug-resistant gram-negative bacteria 4, 5. The 2016 IDSA/ATS guidelines specifically recommend intravenous polymyxin B (colistin or polymyxin B) for HAP/VAP caused by Acinetobacter species or carbapenem-resistant pathogens sensitive only to polymyxins 4.
For carbapenem-resistant Pseudomonas aeruginosa (CRPA), polymyxin B resistance rates remain relatively low at 1.3-3.3%, though slightly higher in MDR strains 3. Recent surveillance data from China showed 94.1% of gram-negative bloodstream isolates remained susceptible to polymyxin B 6.
Important Caveats:
- Polymyxin B loses 50% of its activity in the presence of serum, resulting in lower active blood levels 1
- The drug does not cross the blood-brain barrier; for meningeal infections, intrathecal administration is required 1
- Tissue diffusion is generally poor 1
- Resistance, though still uncommon, is emerging and should be monitored through susceptibility testing 7