Comparative Analysis of Colistin Susceptibility Using Micronaut MIC-Strip, Disc Elution, and Broth Microdilution in MDR Gram-Negatives.

Background: Rapid, simple, and accurate methods are needed to detect colistin susceptibility in multidrug-resistant Gram-negative isolates. Both EUCAST and CLSI recommend broth microdilution (BMD) for antimicrobial susceptibility testing of colistin, but BMD is rarely used in routine microbiology laboratories. In search of alternative, more practical methods, some commercial kits were propagated, along with CLSI's recommendation of using Colistin disc elution method. In this study, we aimed to compare colistin susceptibility testing both by MMS, which is a commercial BMD product, and by Colistin disc elution method to BMD using multidrug-resistant Acinetobacter baumanii, Pseudomonas aeruginosa, and Klebsiella pneumoniae.

Methods: Multidrug-resistant Acinetobacter baumannii, P. aeruginosa, and K. pneumoniae isolates detected from various clinical samples in Marmara University Pendik Training and Research Hospital´s clinical microbiology laboratory between January 1, 2021, and July 30, 2022, were included in the study. Colistin susceptibilities were determined using the CBDE method, Micronaut MIC Strip colistin assay (MMS), and BMD. The results were compared with BMD as the reference method. Categorical agreement (CA), essential agreement (EA), major error (ME), and very major error (VME) rates were calculated.

Results: The study included 185 multidrug-resistant A. baumannii, P. aeruginosa, and K. pneumoniae isolates from various clinical samples. Out of these isolates, 13 (6.5%) were found to be resistant. The MMS test demonstrated a categorical agreement (CA) of 100% for A. baumannii and P. aeruginosa, while the CBDE method achieved a CA of 100% only with A. baumannii.

Conclusions: Both the MMS and CBDE tests are compatible with the BMD method. The CBDE, which does not require specialized equipment or advanced techniques, may be more suitable for laboratories with limited resources. In contrast, laboratories with greater financial means may find the MMS test to be advantageous in providing an accurate minimum inhibitory concentration.