Identification of Plasmid-Mediated Colistin Resistance in Multidrug-Resistant Gram-Negative Rods Isolated From Immunocompromised Patients

Introduction: Antimicrobial resistance (AMR) poses a catastrophic threat to immunocompromised patients, particularly through the spread of multidrug-resistant (MDR) Gram-negative bacteria. With the rise of carbapenem resistance, colistin has returned as a last-resort therapy. However, the emergence of the mobilized colistin resistance (mcr) gene threatens this final defense. This study evaluates the prevalence of phenotypic colistin resistance and the mcr-1 gene in clinical isolates from immunocompromised patients. Methods: This descriptive cross-sectional study was conducted over 12 months at Lahore General Hospital and the University of Health Sciences, Lahore. We collected samples of 100 MDR Gram-negative rods (GNRs) from biological specimens (urine, blood, pus, wound, tracheal secretions) of immunocompromised patients. Identification was confirmed via API 20E (Analytical Profile Index). Antibiotic susceptibility was determined using the Kirby-Bauer method. Crucially, colistin susceptibility was assessed using the CLSI-recommended colistin broth disk elution (CBDE) method. Polymerase chain reaction (PCR) was used to screen for mcr-1 through mcr-5 genes. Results: The sample comprised 100 isolates, predominantly Escherichia coli (32%) and Klebsiella pneumoniae (25%). Phenotypic screening revealed a colistin resistance rate of 6% (6/100). Resistance was highest in K. pneumoniae (16%) and Acinetobacter baumannii (7.7%). Notably, phenotypic resistance was not observed in Escherichia coli or Enterobacter cloacae in this specific cohort. Molecular analysis confirmed that 100% (6/6) of the colistin-resistant isolates harbored the mcr-1 gene. No isolates carried mcr-2, -3, -4, or -5. High resistance rates were observed against cephalosporins and carbapenems. Conclusion: While the overall prevalence of colistin resistance (6%) is relatively low compared to carbapenems, the universal presence of mcr-1 in resistant strains suggests a plasmid-mediated transmission route that poses a severe risk to immunocompromised populations. The CBDE method offers a reliable diagnostic alternative for resource-limited settings.