Comparison of BD Phoenix semi-automated system and liquid microdilution methods in detecting colistin resistance in Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii clinical isolates.

Objective-Aim: Colistin susceptibility tests have gained importance as the need for colistin use increases in multidrug-resistant (MDR) bacterial infections. The aim of this study was to compare the minimal inhibitor concentration (MIC) of colistin detected by automated system with MICs determined by broth microdilution (BMD) in multidrug resistant A.baumannii, P. aeruginosa, K. pneumoniae and E. coli isolates. Materials and Methods: A. baumanni (92 isolates), K. pneumoniae (68 isolates), P. aeruginosa (55 isolates) and E. coli (21 isolates) strains which were isolated from clinical specimen sent to Dicle University Hospitals microbiology laboratory were included in the study. Antimicrobial susceptibility tests (AST) of the isolates were studied with the BD Phoenix 100 (Becton Dickinson, USA) automated system by using the Phoenix UNMIC-401 / ID Panel and NMIC-400 / ID panels, for urine and other samples, respectively. The isolates were also studied by BMD method which was taken as the reference method. Colistin sensitivities of Phoenix 100 system were compared with those obtained with BMD system. Results: Of the 236 isolates included in the study, 65 were isolated from tracheal aspirate, 64 from urine, 51 from blood, 44 from wounds, 4 from abscess, 4 from drains, and 4 from pleural samples. Colistin resistance was found in 36 (15.3%) of 236 isolates. 15.2%, 20.6%, 5.5% and 23.8% colistin resistance were determined in A. baumannii, K. pneumoniae, P. aeruginosa and E. coli isolates, respectively. The Phoenix 100 system has a very major error (VME) rate of 75%; the major error (ME) rate was 0.5%. Conclusion: The results of colistin susceptibility tests in MDR bacterial infections are very important for appropriate antibiotherapy. The Phoenix 100 system was found to be unable to detect colistin-resistant isolates. SMD results should be referenced to determine the sensitivity of newly developed tests or modifications to automated systems. Keywords: Acinetobacter baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, Colistin, Phoenix 100, broth microdilution