Comparison of Broth Microdilution Method with BD Phoenix, Micro Scan and E-test for Carbapenem-resistant Enterobacterales: Colistin Susceptibility Testing

Objective: In the past years, due to the increasing carbapenem resistant Enterobacterales (CRE) infection rates, colistin use has been on the rise. Multi-drug resistant Gram-negative bacteria and colistin resistance are increasing simultaneously; therefore, an accurate method for antimicrobial susceptibility testing of colistin is crucial. Clinical and Laboratory Standards Institute and European Committee on Antimicrobial Susceptibility Testing recommend the standard broth microdilution (BMD) method for colistin minimum inhibitory concentration testing. In this study, we aimed to examine the performance of BD Phoenix, MicroScan, and E-tests on CRE isolates on the determination of colistin susceptibility. The existing commercial tests were compared to the reference BMD method. Methods: One hundred and twenty non-duplicate clinical Enterobacterales isolates such as Klebsiella pneumoniae (K. pneumoniae), Escherichia coli (E. coli), Enterobacter cloace (E. cloacae) were collected between August 2017 to June 2018. The BD Phoenix, MicroScan systems, and E-tests were used to test colistin susceptibility. Commercial methods were compared with the reference method BMD. Results: Colistin susceptibility was evaluated in 120 Gram-negative clinical isolates, including 108 K. pneumoniae, 10 E. coli, 2 E. cloacae, during the study period. Among the isolates, 66 (55%) were susceptible, and 54 (45%) were resistant to colistin, according to BMD. BD Phoenix, MicroScan, and E-test had 90.90%, 95.45%, and 96.96% sensitivity, respectively, when colistin was tested. Conclusion: In routine clinical practice, the worldwide reference method can hardly be implemented, and commercially available systems are used for the interpretation of colistin susceptibility. Colistin use is increasing for the treatment of multiresistant Gram-negative infections, further and more extensive studies are needed for precise susceptibility testing methods for this compound. We recommend that laboratories use the BMD method at least in selected patient groups in the face of increasing antimicrobial resistance.