Phenotypic Detection of Colistin Resistance Among Multidrug-Resistant Gram-Negative Bacilli

Background: Increasing colistin resistance among multidrug-resistant Gram-negative bacilli is a major concern because colistin remains an important therapeutic option for difficult-to-treat infections. Reliable phenotypic detection is essential for appropriate antimicrobial therapy and surveillance. The objective is to determine the phenotypic profile of colistin resistance among MDR Gram-negative bacilli using broth microdilution and to evaluate feasible phenotypic approaches for its detection. Material and Methods: This prospective, hospital-based, cross-sectional study included 150 non-duplicate MDR Gram-negative bacterial isolates obtained from clinically significant specimens. Organisms were identified using standard microbiological methods, and routine antimicrobial susceptibility testing was performed. Colistin susceptibility was determined using broth microdilution (BMD) as the reference phenotypic method. Colistin resistance was analysed according to bacterial species and clinical specimen. The proposed phenotypic screening method, COL-AS, was designed for comparison with BMD using sensitivity, specificity, predictive values, accuracy and agreement. Results: Of the 150 MDR Gram-negative isolates, K. pneumoniae was the most common (36.0%), followed by E. coli (30.0%), A. baumannii (18.0%) and P. aeruginosa (12.0%). BMD identified 24 (16.0%) isolates as colistin resistant. Resistance was highest among A. baumannii (25.9%), followed by K. pneumoniae (20.4%), P. aeruginosa (16.7%) and E. coli (6.7%). The association between bacterial species and colistin resistance was statistically significant (p=0.041). Conclusion: A considerable proportion of MDR Gram-negative bacilli demonstrated phenotypic colistin resistance, particularly A. baumannii and K. pneumoniae. Reliable phenotypic testing using BMD remains important for accurate detection and surveillance of colistin resistance.

Keywords: Colistin resistance, multidrug-resistant Gram-negative bacilli, broth microdilution, phenotypic detection, antimicrobial susceptibility testing, Acinetobacter baumannii, Klebsiella pneumoniae.