Estimation of Colistin Resistance Among Multidrug-Resistant Gram-Negative Bacilli: An Observational Study

Background: The rise in antimicrobial resistance, particularly among Gram-negative bacilli (GNB), poses a significant threat to global public health. Colistin, a polymyxin antibiotic, has re-emerged as a last-resort treatment against multidrug-resistant Gram-negative bacilli (MDR-GNB). However, increasing colistin resistance has become a major concern, especially in clinical settings where treatment options are limited. Aim: To estimate the prevalence of colistin resistance among MDR-GNB and to compare the performance of three phenotypic methods, broth microdilution (BMD), colistin broth disk elution (CBDE), and colistin agar test (CAT), for detecting colistin resistance. Methods: This observational study was conducted over a period of one year at a tertiary care hospital in Gwalior, Madhya Pradesh. A total of 100 MDR-GNB isolates were included as a convenience sample during the study period, which was considered adequate to estimate colistin resistance and evaluate the diagnostic performance of the testing methods. The isolates were screened for colistin susceptibility using BMD (gold standard), CBDE, and CAT as per Clinical and Laboratory Standards Institute (CLSI) guidelines. Diagnostic performance, including sensitivity, specificity, and categorical agreement, was assessed for CBDE and CAT in comparison with BMD. Results: Among 100 MDR Gram-negative isolates, Klebsiella pneumoniae (41%), Escherichia coli (35%), Acinetobacter baumannii (13%), and Pseudomonas aeruginosa (11%) were identified. Susceptibility to imipenem was highest in E. coli (20/35; 57.1%), followed by P. aeruginosa (6/11; 54.5%) and A. baumannii (7/13; 53.8%), while K. pneumoniae showed lower susceptibility (13/41; 31.7%). Resistance to ciprofloxacin was observed in 100% of K. pneumoniae, P. aeruginosa, and A. baumannii, and 94.3% (33/35) of E. coli. Resistance to third-generation cephalosporins ranged from 81.8% to 100%. Colistin resistance by BMD was 4% (4/100), with resistance detected in K. pneumoniae (3/41; 7.3%) and A. baumannii (1/13; 7.7%). The CBDE method showed 100% sensitivity, 98.96% specificity, and 99% categorical agreement with BMD, whereas the CAT demonstrated lower sensitivity (75%) and a higher very major error rate (25%). Conclusion: The emergence of colistin-resistant MDR-GNB emphasizes the urgent need for accurate and accessible susceptibility testing. While BMD remains the reference standard, CBDE offers a practical and reliable alternative for routine laboratory use. Regular surveillance and rational use of colistin are essential to curb the spread of resistance and preserve the efficacy of this critical antimicrobial agent.