| Objective The study was conducted to investigate in-vitro activity of tigecycline against common clinical multi-drug resistant bacteria isolated from our hospital in2012, and evaluate different susceptibility testing methods of tigecycline against Acinetobacter baumannii and Klebsiella pneumoniae, to provide reference for clinical in-vitro susceptibility testing of tigecycline.Methods The2,264multi-drug resistant bacteria were analyzed from the Tianjin Medical University General Hospital in2012retrospectively. The bacteria identification and antimicrobial susceptibility tests were performed by Vitek-2compact automatic system and the data were analyzed using WHONET5.4software. The50carbapenem-resietant A.baumannii and49K. pneumoniae strains were collected from the hospital during January to March2012. MIC and inhibitory zone diameters for tigecycline were determined by broth microdilution (BMD), Vitek-2, MIC Test Strip (MTS) and disk diffusion. Data were analyzed by comparing the results from each method to those produced by the reference BMD method.Results (1) The sensitivity rates of tigecycline against methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant coagulase-negative Staphylococd (MRCNS) and vancomycin-resistant Enterococci (VRE) were all100%. The tigecycline MIC50/90of MRSA and MRSCN were both0.125μg/ml, lower than vancomycin and linezolid.(2) According to the FDA and EUCAST breakpoints, the sensitivity rates of ESBLs-producing Escherichia coli, ESBLs-producing Klebsiella pneumoniae, and carbapenem-resistant Acinetobacter baumannii (CRAB) to tigecycline were99.8%/98.4%,86.6%/67.8%,69.0%/35.7%, respectively.(3)For50A.baumannii isolates, the MIC50/90of BMD, Vitek-2and MTS were as follows:1/2μg/ml,2/4μg/ml and1.5/3μg/ml, respectively. For49K. pneumoniae isolates, the MIC50/90of BMD, Vitek-2and MTS were as follows:1/2μg/ml,0.5/1μg/ml and0.5/1μg/ml, respectively. According to FDA/EUCAST standards, the sensitivity rates of A.baumannii determined by BMD, Vitek-2, MTS were94.0%/70.0%,68.0%28.0%,90.0%/44.0%, respectively, and the sensitivity rates of K. pneumoniae were91.8%/75.5%,91.8%/91.8%,91.8%/91.8%. The tigecycline sensitivity rates of A.baumannii and K.pneumoniae were0to96.0%and63.3%to95.9%, which detected by disk diffusion method adopted four different breakpoints.(4) According to FDA breakpoints, the categorical agreement (CA) of Vitek-2and MTS with reference method was higher than EUCAST breakpoints. The essential agreement (EA) of A.baumannii and K.pneumoniae determined by Vitek-2was94.0%and95.9%, and the EA determined by MTS was92.0%,83.7%, and both methods produced no very major errors (VME).(5) For A.baumannii, when disk diffusion method using breakpoint≥14mm/≤10mm, there exist the best correlation with BMD, with highest CA (94.0%) and lowest small errors (mE,2.0%). For K.pneumoniae, when disk diffusion method using breakpoint≥16mm/≤12mm, there exist the best correlation with BMD, with highest CA (98.0%) and lowest small errors (mE,2.0%).Conclusion Tigecycline maintained a good antibacterial activity against clinical multidrug-resistant pathogens (except for Pseudomonas aeruiginosa), especially against Gram-positive cocci. The FDA interpretative criteria is more suitable than the EUCAST breakpoints to determine tigecycline susceptibility results, therefore, the FDA interpretative criteria should be applied in clinical. There exist discrepancies between different methods to detect tigecycline susceptibility results. For CRAB, MTS produces better consistence with broth microdilution, while Vitek-2has better correlation with reference method for K.pneitmoniae.There exist less consistence between disk diffusion method and broth microdilution, while the breakpoint should be adjusted according to different bacteria. For CRAB, using the breakpoint as≥14mm/≤10mm has the best consistence with broth dilution method. For Klebsiella pneumoniae, using the breakpoint as≥14mm/≤10mm has the best consistence with reference method. |
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