Identification and Molecular Characterization of Non-fermenting Gram-Negative Bacilli Exhibiting Carbapenem Resistance Isolated From Various Clinical Specimens in a Tertiary Care Hospital

Background: Non-fermenting Gram-negative bacilli (NFGNB), including Pseudomonas aeruginosa and Acinetobacter baumannii, are emerging nosocomial pathogens with intrinsic and acquired resistance to multiple antibiotics. Their increasing association with healthcare-associated infections (HAIs), especially in immunocompromised patients, poses a major therapeutic challenge, particularly due to carbapenem resistance mediated by metallo-β-lactamases (MBLs). This study aimed to isolate and identify NFGNB from various clinical specimens, assess their antimicrobial susceptibility patterns, detect carbapenem resistance phenotypically, and characterize MBL-producing isolates using molecular methods. Methods: A descriptive and retrospective study was conducted in the Department of Microbiology, Government Doon Medical College, Dehradun, India, from September 2020 to February 2022. A total of 3,235 clinical specimens were processed for culture and identification. NFGNB were identified by colony morphology, Gram-staining, and biochemical tests. Antimicrobial susceptibility was performed using the Kirby-Bauer disc diffusion method. Screening for carbapenem resistance was done using imipenem and meropenem antibiotic discs, and MBL production was confirmed by the combined disc test (CDT) and modified Hodge test (MHT). Polymerase chain reaction (PCR) was used to detect blaNDM-1, blaIMP, and blaVIM genes in phenotypically positive isolates. Results: Among 3,235 clinical samples, 1,924 (59.47%) samples showed culture positivity. A total of 220 (25.82%) isolates of NFGNB out of 852 (44.28%) isolates of Gram-negative bacilli were observed. The maximum proportion of NFGNB, excluding Gram-negative bacilli isolates, was reported from the pus (95, 89.62%) sample. Pseudomonas species (121, 55%) and Acinetobacter species (83, 37.22%) were the most common isolates. Imipenem resistance was seen in 96 (43.83%) isolates, and meropenem resistance in 76 (34.54%) isolates. Thirteen isolates (13.54%) were phenotypically MBL-positive, and PCR confirmed MBL genes in all 13 isolates. blaNDM-1 was the most frequently detected gene. High resistance was observed in Acinetobacter isolates to carbapenems and fluoroquinolones, whereas minocycline showed the highest sensitivity. Conclusion: The high prevalence of carbapenem-resistant and MBL-producing NFGNB underscores the urgent need for rapid detection, rational antibiotic use, and stringent infection control measures to limit the spread of multidrug-resistant organisms in healthcare settings.