ABSTRACT
Objective To compare different susceptibility testing methods of tigecycline against Acinetobacter baumannii and Klebsiella pneumoniae.Methods Fifty carbapenem-resietant A.baumannii (CRAB) strains and 49 K.pneumoniae strains were collected from Tianjin Medical University General Hospital during January and March 2012.Minimum inhibitory concentration (MIC) and inhibitory zone diameters for tigecycline were determined by broth microdilution,Vitek-2,MTS and disk diffusion methods.The results of Vitek-2,MTS and disk diffusion methods were compared with those of broth microdilution method.Results According to FDA standards,the susceptibilities of CRAB and K.pneumoniae to tigecycline determined by broth microdilution,Vitek-2 and MTS were 94.0％/91.8％,68.0％/91.8％ and 90.0％/91.8％,respectively.For CRAB isolates,the essential agreement (EA) and categorical agreement (CA) produced by Vitek-2 and MTS were 94.0％/72.0％ and 92.0％/90.0％.MICs determined by Vitek-2 were 1-2 dilutions higher than the reference method in 33 (66.0％) strains,and those determined by MTS were higher in 16 (32.0％) strains and lower in 11 (22.0％) strains.For K.pneumoniae isolates,the EA/CA produced by Vitek-2 and MTS were 95.9％/98.0％ and 83.7％/91.8％,respectively.MICs determined by Vitek-2 were 1-3 dilutions lower than the reference method in 17 (34.7％) strains,and those determined by MTS were 1-3 dilutions lower than the reference method in 39 (79.6％) strains.None of thetwo methods produced very major error (VME) and major error (ME) against two kinds of isolates.The results were determined by disk diffusion method using different breakpoints according different isolates.For CRAB,using ≥14 mm/≤ 10 mm as breakpoint,CA was 94.0％,which was higher than the breakpoint recommended by Jones et al (≥16 mm/≤12 mm,CA was 82.0％) ; and for K.pneumoniae,using the ≥ 14 mm/≤ 10 mm as breakpoint,CA was 93.9％,higher than the FDA Enterobacteriaceae breakpoint (≥ 19 mm/≤ 14 mm,CA was 67.3％).Conclusion For CRAB strains,MTS produces better consistence with broth microdilution,with several higher or lower MIC results.For K.pneumoniae strains,Vitek-2 has better correlation with reference method,with several lower MIC results.The consistence between disk diffusion method and broth microdilution is comparatively lower,and the breakpoint should be adjusted according to different bacteria.