Abstract
<title>Abstract</title> <p> Background <italic>Acinetobacter baumannii</italic> is a leading cause of healthcare-associated infections and is recognized for its remarkable ability to acquire multidrug resistance. The global emergence of carbapenem-resistant <italic>A. baumannii</italic> (CRAB) has become a major therapeutic and public health challenge, primarily driven by the dissemination of carbapenemase-encoding genes. This study aimed to investigate the phenotypic and genomic characteristics of carbapenem resistance among multidrug-resistant (MDR) <italic>A. baumannii</italic> isolated from clinical specimens and corresponding patient environmental surroundings. Methods A prospective hospital-based study was conducted from August 2023 to October 2024 at a tertiary care hospital in Mysuru, India. A total of 110 non-duplicate MDR <italic>A. baumannii</italic> isolates, comprising 55 clinical and 55 environmental isolates, were included. Species identification and antimicrobial susceptibility testing were performed using standard microbiological methods and the VITEK® 2 automated system. Whole-genome sequencing was performed on all isolates to identify carbapenemase and β-lactamase genes. Genome assembly, annotation, and antimicrobial resistance gene identification were carried out using established bioinformatics pipelines, including AMRFinderPlus. Results Carbapenemase-associated genes were highly prevalent among both clinical and environmental isolates. The acquired carbapenemase gene <italic>blaOXA-23</italic> was detected in all isolates (110/110; 100%), while <italic>blaOXA-66</italic> and <italic>blaNDM-1</italic> were identified in 98 (89.1%) and 94 (85.5%) isolates, respectively. The cephalosporinase gene <italic>blaADC-73</italic> was detected in 41 (74.5%) clinical isolates and 35 (63.6%) environmental isolates. Additional β-lactamase genes, including <italic>blaADC-30</italic> , <italic>blaTEM-1</italic> , <italic>blaDHA</italic> , <italic>blaOXA-68</italic> , <italic>blaADC-76</italic> , <italic>blaPER-7</italic> , <italic>blaOXA-69</italic> , and <italic>blaADC-191</italic> , were detected at lower frequencies. Clinical isolates demonstrated a higher prevalence of <italic>blaADC-73</italic> , whereas <italic>blaADC-30</italic> , <italic>blaTEM-1</italic> , and <italic>blaDHA</italic> were comparatively more frequent among environmental isolates. Conclusions The universal presence of <italic>blaOXA-23</italic> , together with the high prevalence of <italic>blaOXA-66</italic> and <italic>blaNDM-1</italic> , highlights the widespread dissemination of carbapenem resistance determinants among MDR <italic>A. baumannii</italic> circulating in both clinical and hospital environmental settings. These findings underscore the importance of continuous genomic surveillance, robust infection prevention and control measures, and antimicrobial stewardship programmes to limit the spread of carbapenem-resistant <italic>A. baumannii</italic> within healthcare facilities. </p>