Investigation of carbapenemase gene distribution in Acinetobacter baumannii in a region with high carbapenem resistance and detection of a silent NDM outbreak.

Carbapenem resistance driven by carbapenemase production in Gram-negative bacteria is a global threat. Carbapenem-resistant Acinetobacter baumannii (CRAb) is a critical priority pathogen listed by the World Health Organization (WHO). While blaOXA-23 and blaOXA-24/40-like are most frequently reported in CRAb, an increasing prevalence of blaNDM has been noted in recent years. This study aimed to investigate the distribution of carbapenemase genes among CRAb isolates obtained from three tertiary care centres in Türkiye.A total of 336 CRAb isolates recovered from various clinical specimens (blood, respiratory samples, wound/tissue swabs, urine, and catheter tip samples) between January 2023 and October 2024 were included (Centre-1:198; Centre-2:86; Centre-3:52). Antimicrobial susceptibility testing was performed using the VITEK2 or BD Phoenix system, with additional tests carried out using the disc diffusion or broth microdilution methods. The results were interpreted in accordance with EUCAST criteria. The presence of blaOXA-51, blaOXA-23, blaOXA-24/40-like, blaOXA-58, blaVIM, blaIMP, blaNDM and blaKPC was tested by qPCR. The clonal relationship among the isolates producing metallo-β-lactamase (MBL) was assessed using AP-PCR and PFGE methods.Our study detected blaOXA-23 in 305 isolates (90.8%), blaOXA-24/40-like in 88 (26.2%), blaNDM in 21 (6.3%), and blaVIM in one isolate. However, blaOXA-58, blaIMP and blaKPC were not detected. Dual gene combinations were observed in 73 isolates, while triple combinations were identified in three isolates (two blaNDM/blaOXA-23/blaOXA-24/40-like, one blaVIM/blaOXA-23/blaOXA-24/40-like). AP-PCR/PFGE demonstrated a clonal relationship among 18 isolates, all originating from a single intensive care unit at Centre-1. Because carbapenem resistance is endemic in this unit, the cluster was indistinguishable from the background resistant population and was detected only retrospectively by molecular typing; we therefore describe it as a silent outbreak.In this study blaOXA-23 and blaOXA-24/40-like were the predominant carbapenemase genes in CRAb, with blaNDM detected at a notable frequency. The detection of combined gene profiles and inter-centre differences highlight the potential impact of local antibiotic use and infection control practices on the dissemination of resistance, and the clonal cluster identified here shows that such dissemination can go unrecognised in endemic settings.