In vitro activity of tigecycline,minocycline and polymyxin B against Acinetobacter baumannii with decreased susceptibility or nonsusceptibility to carbapenems

Objective The in vitro activity of tigecycline,minocycline,polymyxin B and other comparators against Acinetobacter baumannii with decreased susceptibility or nonsusceptibility to carbapenems was compared for better clinical management of such infections.Methods Agar dilution method was used to determine the minimum inhibitory concentrations(MICs)of 15 antimicrobial agents against A.baumannii collected during the period from2002 to 2009 in Shenzhen People′s Hospital.A total of 56 strains were found with decreased susceptibility to carbapenems,which was defined as imipenem MIC=1 to 4 mg/L and meropenem MIC=1 to 4 mg/L.Overall,178 strains were identified as carbapenem-non-susceptible A.baumannii,which was defined as imipenem MIC ≥ 8 mg/L or meropenem MIC ≥ 8 mg/L.The data were analyzed by WHONET5.6 software. Results Polymyxin B showed the highest activity against the A.baumannii strains with decreased susceptibility or nonsusceptibility to carbapenems.All the strains were susceptible to polymyxin B in vitro.Both MIC50 and MIC90 values were 1 mg/L. Tigecycline and minocycline could inhibit about 80% of the A.baumannii strains with decreased susceptibility to carbapenems.The MIC50 /MIC90 ratio was 4/8 mg/L and 8/ 16mg/L,respectively.About 95% of the carbapenem-nonsusceptible A.baumannii strains were susceptible or intermediate to tigecycline and minocycline.The MIC50 /MIC90 ratio was 4/4 mg/L and 4/8 mg/L.Conclusions Polymyxin B,tigecycline and minocycline have relatively higher antibacterial activity against the A.baumannii strains with de-creased susceptibility or nonsusceptibility to carbapenems.