Genomic and phenotypic characterization of six multidrug-resistant Acinetobacter pittii isolates

The Acinetobacter calcoaceticus-baumannii complex is a significant cause of nosocomial infections. While A. baumannii is the most common and clinically significant member globally, recent studies suggest that A. pittii is an emerging pathogen within the complex. Clinical A. pittii isolates have not been well characterized. In this study, we investigated the genomic and phenotypic characteristics of six clinical A. pittii isolates from five healthcare facilities. All isolates were resistant to ceftazidime and ertapenem, and one was resistant to imipenem and meropenem. Using whole genome sequencing and phylogenetic analysis, we found that isolates were not linked genetically, harbored OXA-type β-lactamase genes, and exhibited a wide array of virulence factors. Plasmids were identified in two isolates. Transfer of a plasmid carrying blaOXA-72 from AP290R to a carbapenem-susceptible A. baumannii recipient strain conferred carbapenem resistance. A. pittii isolates varied in motility and biofilm production. When tested in a Zophobas morio survival model, the carbapenem-resistant strain AP290R showed high virulence and led to increased mortality, reaching 70% within 24 h post-infection. This study highlights the potential of A. pittii to serve as a reservoir of carbapenem resistance and cause severe infections. Infection control efforts to limit the spread of Acinetobacter calcoaceticus-baumannii complex should include A. pittii.