Polymyxins: older antibiotics for a new threat.

Polymyxin B and colistin have demonstrated reasonable success in infections due to multidrug-resistant A. baumannii and P. aeruginosa. Polymyxins are only recommended for highly selected cases of serious infections caused by gram-negative bacilli resistant to currently available antibiotics to avoid their potential toxicity. Pharmacodynamic studies are still needed to determine appropriate dosage regimens that will optimize effectiveness and minimize toxicity.