Detection of colistin resistance via four methods in multidrug-resistant Gram-negative rods isolated from blood cultures.

Abstract

The broth microdilution (BMD) method recommended for the detection of colistin resistance is labor-intensive, time-consuming, and difficult to apply in routine laboratories. Thus, various methods, such as disk elution, commercial microdilution, and rapid polymyxin-NP tests have been developed for the 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 for the detection of colistin resistance, and compared with the reference method. 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. 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 a 100% synergistic effect, but the colistin-tigecycline combination showed an 80% synergistic effect and 20% indifference effect. 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.