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

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.

This study aimed to screen for colistin resistance and to evaluate the antimicrobial susceptibility patterns among gram-negative bacteria isolated from clinical samples.

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.

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.

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.