Differences in potency and categorical agreement between colistin and polymyxin B when testing 15,377 clinical strains collected worldwide.

Gram-negative bacilli (n=15,377) were tested against colistin (polymyxin E) and polymyxin B by a commercial broth microdilution method (Sensititre®). Among Pseudomonas aeruginosa and Acinetobacter spp., colistin and polymyxin B MIC values were within ±1 doubling dilution for >99.0% of strains. Among Klebsiella spp. and Escherichia coli, 55.0 and 53.2% of strains displayed a colistin MIC 2-fold lower than polymyxin B, but polymyxin B was slightly more potent than colistin against strains with decreased susceptibility to either polymyxin.