Polymyxin is NOT an Antifungal Agent
Polymyxin is an antibiotic specifically active against Gram-negative bacteria, not fungi. It is a polypeptide cationic antibiotic used to treat carbapenem-resistant Gram-negative bacterial infections (CRGNB), including carbapenem-resistant Enterobacteriaceae (CRE), carbapenem-resistant Acinetobacter baumannii (CRAB), and carbapenem-resistant Pseudomonas aeruginosa (CRPA) 1, 2.
Mechanism and Clinical Use
Polymyxins (polymyxin B and polymyxin E/colistin) target lipid A, the membrane-embedded lipopolysaccharide (LPS) anchor on Gram-negative bacterial cell surfaces 3. This Gram-negative specificity explains why polymyxins have no role in treating fungal infections in standard clinical practice.
Clinical guidelines consistently position polymyxins as antibacterial agents:
- Recommended for CRGNB infections, preferably as combination therapy over monotherapy 1
- Used for severe Acinetobacter baumannii infections resistant to beta-lactams, fluoroquinolones, and tigecycline 2
- Reserved as last-line therapy for extensively drug-resistant Gram-negative bacteria 4
Research Context vs. Clinical Reality
While in vitro research studies have demonstrated that polymyxin B can exhibit antifungal activity against certain fungi (Candida, Cryptococcus, Fusarium, Aspergillus) at relatively high concentrations 5, 6, 7, this does not translate to clinical use. These findings are:
- Limited to laboratory settings with concentrations often exceeding clinically achievable or safe levels
- Not supported by clinical trials or guidelines
- Not FDA-approved for antifungal indications
- Potentially synergistic with azole antifungals in vitro, but without clinical validation 8, 7
Common Pitfall to Avoid
Do not confuse polymyxin's in vitro antifungal effects with clinical utility. The research showing antifungal properties is exploratory and does not change clinical practice. When treating fungal infections, use established antifungal agents (amphotericin B, azoles, echinocandins) as recommended by infectious disease guidelines 9.
For invasive fungal infections in neutropenic or critically ill patients, empirical or preemptive antifungal therapy should follow established protocols with proven antifungal agents, not polymyxins 9.