Co-occurrence and genetic basis of carbapenem and colistin resistance in multidrug-resistant gram-negative Bacilli

OBJECTIVES: This study investigated the prevalence of carbapenem and colistin resistance among Gram-negative bacilli (GNB) isolated from clinical specimens in Rangamati, Bangladesh, and characterized major carbapenemase genes (blaNDM, blaVIM, blaKPC, blaOXA-48, blaIMP) and colistin resistance determinants (phoP, phoQ, pmrA, pmrB, pmrC, mcr-1) to support local antimicrobial stewardship and infection control efforts. METHODS: A cross-sectional study was conducted on 305 clinical specimens obtained from 305 admitted patients, processed by using standard microbiological methods. Antimicrobial susceptibility testing was performed by modified Kirby Bauer disc diffusion, while colistin minimum inhibitory concentrations were determined by MIC testing. Carbapenemase production was assessed phenotypically using the modified carbapenem inactivation method. Polymerase chain reaction assays were used to detect carbapenemase and colistin resistance genes. Data were analyzed using SPSS version 26, and ethical approval was obtained. RESULTS: Of the 305 specimens, 190 (62.3%) were culture positive, yielding 198 isolates. Gram-negative bacilli accounted for 163(82.3%) of isolates. The most frequently identified pathogens were Klebsiella spp. (29.3%), Pseudomonas spp. (27.8%), Escherichia coli (17.2%), and less frequently isolated GNB included Enterobacter spp. (4.0%), Proteus spp. (2.5%), Salmonella spp. (1.5%). Among GNB, carbapenemase genes were detected in 39 (23.9%) isolates by molecular methods, while 37 (22.7%) were positive by phenotypic testing. Colistin resistance was observed in 9 (5.5%) isolates. blaNDM was the predominant carbapenemase gene, whereas blaIMP was not detected. Among colistin-resistant isolates, phoQ was the only resistance determinant identified. Co-resistance to carbapenems (associated with blaNDM) and colistin was found in 3.2% of isolates. CONCLUSION: Carbapenem resistance among Gram-negative bacilli in Rangamati is substantial and largely driven by blaNDM. Although colistin resistance remains low, the presence of co-resistant isolates highlights the need for ongoing molecular surveillance and strengthened antimicrobial stewardship.