Evaluation of Colistin and Tigecycline Susceptibility Testing Methods for Klebsiella pneumoniae and Acinetobacter baumannii Clinical Isolates

Background: The evolution of nosocomial infections by multidrug resisitance (MDR) and extensive drug resistance (XDR) Acinetobacter baumannii and klebsiella pneumoniae are considered a major health problem owing to the relatively limited treatment options. Colistin and tigecycline are increasingly used as a last choice for treatment of these infections. The most accurate antibiotic susceptibility methods for colistin and tigecycline are still challenging. Objectives: The aim of the current study was to detect colistin and tigecycline antibiotic susceptibility of K. pneumoniae and A. baumanii and evaluate disk diffusion (DD), E- test and VITEK 2 automated system compared to broth dilution (BD) test. Methodology: This study was performed on 35 K. pneumoniae and 15 A. baumanii clinical isolates collected from patients admitted to Benha University Hospitals. The isolated strains were identified by the standard laboratory technique with subspecies identification by VITEK 2 automated system. Colistin and tigecycline antibiotic susceptibility for K. pneumoniae and A. baumanii were evaluated by E-test, disk diffusion and VITEK 2 compared to BD as the reference method. Results: Through the study of the studied k. pneumoniae and A. baumanii strains, The essential and categorical agreements of colistin suscebtibility were (82% & 80 %) for E-test, (92% & 98%) for VITEK 2 and categorical agreement for DD was 54%. The essential and categorical agreements of tigecycline suscebtibility were (96% & 98%) for E-test, (88% &78%) for VITEK 2 and categorical agreement for DD was 74%. Conclusion: For colistin, VITEK 2 is considered a reliable method to detect colistin susceptibility while E-test and disk diffusion showed a poor performance. For tigecycline, E-test showed the best performance compared to the gold standard test while shortcomings of automated VITEK 2 and manual DD were observed.