Comparison of Modified Colistin Broth Disk Elution, Automated System, and Broth Microdilution Results in Determining Colistin Susceptibility in Multidrug-Resistant Gram-Negative Bacteria

Colistin is an antibiotic used to treat infections caused by multidrug-resistant (MDR) gram-negative bacteria. However, the broth microdilution (BMD) method, the reference standard for determining colistin susceptibility, is labor-intensive and time-consuming, prompting the development of alternative methods. This study aimed to compare the automated system and the modified colistin broth disk elution (CBDE) method with the BMD method for determining colistin susceptibility. A total of 161 MDR gram-negative bacterial isolates from clinical samples obtained between April and December 2024 were analyzed. Colistin susceptibility was assessed using the Phoenix™ M50 automated system, BMD, and the modified CBDE method. For Enterobacterales isolates, when the modified CBDE test results were compared with the BMD method results, the categorical agreement (CA), very major error (VME), and major error (ME) rates were determined to be 95.8%, 2.1%, and 2.1%, respectively. The Phoenix™ M50 system showed a CA of 94.7%, with VME and ME rates of 2.1% and 3.1%, respectively. For non-fermenting gram-negative bacteria, the modified CBDE test achieved 100% CA for Pseudomonas aeruginosa, the CA, VME, and ME rates for Acinetobacter baumannii isolates were found to be 97.5, 0, and 2.5%, respectively. The Phoenix™ M50 system showed 100% CA for P. aeruginosa and a CA of 95%, with VME and ME rates of 2.5% and 2.5% for A. baumannii, respectively. In conclusion, the modified CBDE method offers a simple, cost-effective, and reliable alternative for colistin susceptibility testing, providing results comparable to BMD. It can be considered a viable screening method.