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Activity of colistin against Pseudomonas aeruginosa: inhibition by calcium.

Human serum inhibits the antibacterial activity of colistin and polymyxin B against Pseudomonas aeruginosa in vitro. By use of a colony-count assay, the inhibitory factor in serum was identified as calcium. The bactericidal activity of colistin was inhibited by either lO"3 m CaCIs, 1596 human serum, or 1596 heparinized plasma. Colistin was not inhibited by plasma containing chelating agents. Colistin may be less active in vivo than predicted on the basis of in-vitro tests of susceptibility to the antibiotic.