• Thean Yen Tan, Lily Siew Yong Ng, Eugene Tan, and Gary Huang.
• Laboratory Medicine Services, Changi General Hospital, 2 Simei Street 3, Singapore 529889. thean_yen_tan@cgh.com.sg
• J. Antimicrob. Chemother. 2007 Aug 1; 60 (2): 421-3.

ObjectivesThe study investigated the effect of colistin and minocycline when tested singly and in combination against Acinetobacter baumannii.MethodsThirteen unrelated imipenem-resistant A. baumannii clinical isolates were included in the study. MICs of colistin sulphate and minocycline were determined by broth macrodilution and Etest. Organisms were also tested against the two antibiotics singly and in combination using time-kill methods and an Etest-based method.ResultsNeither colistin nor minocycline when tested alone demonstrated bactericidal activity. However, the combination of colistin and minocycline demonstrated bactericidal activity against most of the isolates tested. At 24 h, the combination of antibiotics demonstrated synergy in 12 of the 13 isolates by time-kill methods. None of the isolates demonstrated synergy by Etest methods.ConclusionsThe combination of colistin and minocycline was found to be bactericidal and synergistic against A. baumannii by time-kill methods. There was no agreement between time-kill and Etest methods for synergy testing.

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