Objectives: This study aimed to combine in vitro phenotyping analysis and whole-genome-sequencing (WGS) to characterise the phenotype and genetic determinants associated with intrinsic resistance in 100 clinical and non-clinical Acinetobacter baumannii strains originating from Germany and Vietnam. Moreover, it aimed to assess whether powdered milk as a food source functions as a potential reservoir of antibiotic resistance and possesses similar antimicrobial resistance (AMR) genes as in clinical strains isolated from Germany. Methods: Antimicrobial susceptibility testing was performed using the broth microdilution method and the minimum inhibitory concentration (MIC) was determined for 18 antibiotics. The WGS data from all isolates were mapped to intrinsic genes known to be associated with phenotypic AMR. Results: The highest resistance frequency was observed for chloramphenicol (100%), followed by fosfomycin (96%) and cefotaxime (95%). The lowest resistant rates were observed for colistin (3%), trimethoprim/sulfamethoxazole (17%), tigecycline (19%), and amikacin (19%). Thirty-five percent of tested strains displayed resistance to at least one of the carbapenems. Resistance to fluoroquinolones, aminoglycosides, tigecycline, penicillins, trimethoprim/sulfamethoxazole, and fourth-generation cephalosporins was determined only in human strains. About one-quarter of isolates (24%) was multidrug-resistant (MDR) and all were of human origin. Among them, 16 isolates were extensively drug resistant (XDR) and 10 from those 16 isolates showed resistance to all tested antibiotics except colistin. In silico detection of intrinsic AMR genes revealed the presence of 36 β-lactamases and 24 non-β-lactamase resistance genes. Two colistin-resistant and 10 ertapenem-resistant strains were isolated from powdered milk produced in Germany. Thirty-eight AMR genes associated with resistance to antibiotics were found in isolates recovered from milk powder. Several resistance mechanisms towards many classes of antibiotics existed in A. baumannii including β-lactamases, multidrug efflux pumps and aminoglycoside-modifying enzymes. Conclusion: The use of WGS for routine public health surveillance is a reliable method for the rapid detection of emerging AMR in A. baumannii isolates. Milk powder poses a risk to contain MDR Acinetobacter strains or resistance genes in Germany.
