Prevalence and molecular characterization of colistin-resistant gram-negative bacteria from a tertiary care hospital in Eastern India

BACKGROUND: Colistin, a last resort antibiotic for multidrug resistant gram-negative infections, is facing enhanced resistance. Data from Eastern India, including Odisha, is limited, highlighting the importance for enhanced surveillance to monitor emerging resistance and guide appropriate empirical therapy. OBJECTIVE: This study aimed to screen for colistin resistance and to evaluate the antimicrobial susceptibility patterns among gram-negative bacteria isolated from clinical samples. METHOD: The study period was from February 2024 to January 2025, during which 366 non-duplicate identified clinical samples were collected from SUM hospital, Bhubaneswar. Antimicrobial susceptibility testing, including colistin tested at three concentrations (10, 25, and 50 µg), was performed using the Kirby-Bauer disk diffusion method. As CLSI and EUCAST guidelines do not recommend disk diffusion for colistin due to poor agar diffusion, the results were considered screening-only and were not used to determine confirmed susceptibility or resistance, and mcr-1 to mcr-9 genes were screened by PCR; however, mcr-1, mcr-3, and mcr-4 were not detected in any of the isolates. Descriptive statistics, including percentages and 95% confidence intervals, were used to summarize prevalence and screening-based colistin non-susceptibility findings. RESULT: Most frequently isolated organisms were E. coli (36.88%) and K. pneumoniae (19.67%). Screen-positive colistin non-susceptibility was observed highest in K. pneumoniae (59.72%), P. aeruginosa (29.62%) and E. coli (8.14%). Multiple mcr genes were detected among screen-positive isolates, with K. pneumoniae positive for mcr-2, 5, 6, 7, 8, 9, E. coli was positive for mcr-5 and mcr-8, A. baumannii for mcr-2, 6, 7 and mcr-1, mcr-3, and mcr-4 were absent in all isolates; P. aeruginosa harboured mcr-7 & mcr-9 only. K. pneumoniae, E. coli and A. baumannii showed high multidrug resistance patterns. These findings are based on screening-positive results only. Prevalence estimates of screening-based colistin non-susceptibility were interpreted descriptively, and 95% confidence intervals were calculated for bacterial isolates and clinical sample sources where appropriate. CONCLUSION: This study highlights screen-positive colistin non-susceptibility among gram-negative isolates in Odisha, India. These screening-based observations underscore the need for confirmatory testing using broth microdilution and ongoing molecular surveillance.