In-vitro activity of tigecycline against clinical isolates of Acinetobacter baumannii in Taiwan determined by the broth microdilution and disk diffusion methods

A total of 393 isolates of A. baumannii were collected from patients treated at 19 teaching hospitals in Taiwan. Minimum inhibitory concentrations (MICs) and inhibitory zone diameters for tigecycline were determined by the broth microdilution method and the disk diffusion method, respectively. The MIC results were interpreted using the US FDA tigecycline susceptibility breakpoints for Enterobacteriaceae (susceptible [S] ≤2 μg/mL; intermediate [I] 4 μg/mL; resistant [R] ≥8 μg/mL). The disk diffusion results were interpreted by criteria recommended by Jones et al. (S ≥16 mm; I 13-15 mm; R ≤12 mm) and also by those recommended by the US FDA for Enterobacteriaceae (S ≥19 mm; I 15-18 mm; R ≤14 mm). The percentages of susceptible, intermediate and resistant isolates determined by the broth microdilution method were 80.9%, 12.2%, and 6.9%, respectively. The rates of susceptible, intermediate and resistant isolates by the disk diffusion method using the criteria of Jones et al. were 88.3%, 9.9% and 1.8% and using the US FDA criteria were 44.0%, 51.7% and 4.3%. Using the criteria recommended by Jones et al., the total error rate of the disk diffusion method in comparison with the broth microdilution method was 14.2% (56/393). For routine susceptibility testing of tigecycline against A. baumannii the broth microdilution method, not the disk diffusion method, should be used due to the poor correlation of results between these two methods interpreted either by the Jones et al. or US FDA criteria.