Detection of colistin resistance via four methods in multi-drug resistant gram negative rods isolated from blood cultures

Introduction: The broth microdilution (BMD) method recommended for the detection of colistin resistance is labor-intensive and time-consuming, and it is difficult to apply in routine laboratories.. Thus, various methods, such as disk elution, commercial microdilution and rapid polymyxin-NP tests have been developed for detection of colistin resistance. In this study, a total of 102 multi-resistant Gram-negative bacteria isolated from blood cultures were evaluated by four different methods to detection of colistin resistance, and compared with the reference method.Methodology: For the detection of the compatibility of these methods with the reference method, categorical and essential agreements, very major, major and minor error rates were determined. Colistin-tigecycline and colistin-meropenem combinations were investigated in colistin-resistant isolates.Results: Of the isolates, 15 (15%) [K. pneumoniae (n=12), A. baumannii (n=2), E. coli (n=1)] were resistant to colistin with reference BMD method. MIC50 and MIC90 values of all isolates were ≤0.25 μg/ml and 16 μg/ml, respectively. The categorical agreement rates were 100% for commercial microdilution, disk elution and RPNP test. The essential agreement rates of commercial microdilution, disk elution and broth macrodilution were 78.4%, 86.3% and 100%, respectively. Although there were no major errors in these methods, the macrodilution (12%) and commercial microdilution (20.6%) methods showed the most minor errors. Colistin-meropenem combination showed 100% synergistic effect, but colistin-tigecycline combination showed 80% synergistic effect and 20% indifference effect.Conclusions: Disk elution and RPNP tests are suitable for routine use because they are the most efficient, easiest, low-cost and good performance tests in detecting colistin resistance