Colistin is a complex antibiotic containing greater than 30 components, with the cyclic polypeptide antibiotics polymyxins E (colistin A) and E (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 (Ks = 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.