Antimicrobial susceptibility testing of colistin: Evaluation of standard broth microdilution method for gram-negative bacilli

Background: Colistin is regarded as the last resort for managing infections caused by multidrug-resistant (MDR) Gram-negative bacilli (GNB). The World Health Organization (WHO) includes colistin on its list of critically necessary antimicrobials. Minimum inhibitory concentrations (MICs) are used to monitor the development of colistin resistance. This study aimed to assess the performance of the Broth Microdilution Method (BMD) against routine Kirby-Bauer disk diffusion (KBDD) and automated BD Phoenix for the detection of the in vitro activity of colistin against GNB. Methods: A cross-sectional study was conducted in the Department of Microbiology, LLRM Medical College, Meerut, Uttar Pradesh, from September 2023 to January 2024. The KBDD method, BMD method, and BD Phoenix (Becton Dickinson, USA) automated system were used to detect colistin susceptibility in 320 GNB isolated from various clinical samples. MIC determined by the BMD method was interpreted according to Clinical Laboratory Standards Institute (CLSI) 2023 guidelines. Results: In our study, 320 isolates of GNB were identified from patients with a mean age of 45.34 years. A total of 320 isolates [145 (45.31%) Escherichia coli, 124 (38.75%) Klebsiella pneumoniae, 32 (10.0%) Pseudomonas aeruginosa, and 19 (5.93%) Acinetobacter baumannii complex] were tested simultaneously with all three methods for colistin susceptibility. The overall resistance to colistin among GNB was found to be 17.18% by the gold standard BMD method, 15.31% by BD Phoenix, and 14.37% by KBDD. Conclusion: BMD is the most cost-effective, authentic method for routine testing of colistin susceptibility as compared to other methods. The comparative analysis revealed that BMD is superior to other methods in detecting colistin susceptibility, emphasizing its potential role in guiding clinicians in antibiotic therapy decisions.