Detecting of colistin resistance via different methods in multi-drug resistant gram negative bacilli isolated from blood cultures

The isolation of colistin-resistant bacteria from bloodstream has increased for few years, causing therapy failure. It is recommended to use colistin in combination in infections caused by these bacteria. Broth microdiution method is the recommended as a standard method to detect colistin resistance. However this method could not be used in routin laboratories since it is labor-intensive and time consuming; therefore the requirement for new easy methods has emerged. In this study, susceptibility to colistin of MDR-GNB was studied by macrodilution, disc elution, commercial microdilution, rapid polymyxin NP tests and compared to broth microdiution to evaluate detecting of colistin resistance in MDR-GNB (n=102) isolated from blood cultures sent from different wards in 2019-2020. The combination of colistin with tigecycline and meropenem were then tested by checkerboard method. Additionally mcr-1 gene was investigated by PCR in colistin resistant isolates. Resistance to colistin was found to be 15%, MİK50 and MİK90 were found to be ≤0.25 μg/ml and 16 μg/ml by BMD method. The categorical agreement was very well (100%) in four methods no very major errors. However macrodilution and commerical microdilution (Diagonistics 8011) methods showed the highest minor errors rates amongst the four methods. The best essential agreement rate was detected by disc elution method. Colistin-meropenem combination exhibited 100% synergism and colistin-tigecycline 80%. No mcr-1 genes were detected in colistin resistant isolates. In conclusion, disc elution and HPNP which those newly developed methods for detecting resistance to colistin were the easiest, cheapest, most efficient and best performing methods, so they are convenient to use in routine laboratory. Colistin-meropenem combination is thought to be an excellent choice to treat bacteremia/sepsis caused by colistin-resistant bacteria.