Colistin

Colistin is a complex antibiotic containing greater than 30 components, with the cyclic polypeptide antibiotics polymyxins E1 (colistin A) and E2 (colistin B) as the major components. It was originally isolated from B. polymyxa. Colistin inhibits the growth of the Gram-negative bacteria E. coli, P. aeruginosa, P. fluorescens, and S. enterica (MICs = 0.04-2.08 μg/ml) and Gram-positive L. ivanovii and L. monocytogenes (MICs = 2.5-10 μg/ml) but is not active against Gram-positive L. lactis, P. polymyxa, P. acidilactici, or S. aureus at concentrations up to 5 μg/ml. It also inhibits the growth of clinical isolates of both susceptible and multidrug-resistant P. aeruginosa (MICs = 1-2 mg/l). Colistin binds selectively to LPS from susceptible strains of K. pneumoniae compared to resistant strains (Kis = 0.56 and 2.83 μM, respectively), which may contribute to its mechanism of action against Gram-negative bacteria. In vivo, colistin slows the growth of P. aeruginosa and A. baumannii in a neutropenic mouse model of thigh infection.