Evaluation of Broth Disk Elution Method to Determine Colistin Resistance in Klebsiella pneumoniae and Escherichia coli Strains.

BACKGROUND

The reference broth microdilution (rBMD) method for the determination of colistin resistance is very laborious and time consuming, and many manual errors can occur. There are also limitations in detection of colistin heteroresistance. Therefore, alternative methods with satisfactory performance are required for routine laboratory work. In our study, the colistin broth disk elution (CBDE) method recommended by the Clinical and Laboratory Standards Institute (CLSI) for the detection of colistin resistance in routine applications was compared with rBMD. The compatibility and error rates of the method were evaluated and its usability in routine laboratory studies was examined.

METHODS

Eighty-nine multidrug resistant Klebsiella pneumoniae and five Echerichia coli strains isolated from various clinical specimens were included in the study. Identification of strains and antibiotic susceptibility tests were performed with MALDI-TOF MS (bioMerieux, France) and Vitek-2 (bioMerieux) system. Minimum inhibitory concentration (MIC) was studied in 0.125 - 128 mg/L dilution range by using polystyrene microplate and colistin sulfate salt according to ISO-standard (20776-1) recommendations for the reference BMD test. The CBDE method was performed according to the CLSI recommendations. Isolates with MIC ≤ 2 mg/L were considered susceptible, while isolates with MIC > 2 mg/L were considered resistant according to EUCAST recommendations. The performance of the CBDE method was evaluated according to ISO criteria (Categorical agreement > 90%; major error and very major error rates < 3%).

RESULTS

Categorical agreement for all 58 and 36 isolates found to be resistant and susceptible, respectively, by rBMD was found to be 100% with CBDE test. Since < 1 and > 4 µg/mL values could not be determined with the CBDE method, essential agreement (EA) could not be calculated. No major or very major errors were detected.

CONCLUSIONS

Our results showed that the performance of the CBDE test is good when compared to the rBMD method. According to our data, we believe that the CBDE method can be used in routine laboratories for the detection of colistin resistance.