Contributions of plasmid p1AB5075-encoded antibiotic resistance genes to multidrug resistance of Acinetobacter baumannii AB5075

Infections with multidrug resistant Acinetobacter baumannii are considered a threat to human and animal health. The widely studied A. baumannii strain AB5075 displays a high degree of antibiotic resistance. In this study, we experimentally validated that antibiotic resistance is largely mediated by resistance genes located on plasmid p1AB5075. We used a p1AB5075-deficient AB5075 strain to assess individual contributions of p1AB5075-encoded antibiotic resistance genes by ectopically (over-)expressing each gene in the Δp1AB5075 background. By determining individual contributions of seven p1AB5075-encoded antibiotic resistance genes, we show individual and overlapping roles of genes for aminoglycoside resistance and uncover the importance of extended-spectrum ß-lactamase bla _GES-11 for cephalosporin resistance in A. baumannii AB5075. We discovered that aminoglycoside N-acetyltransferase aaC(6’)-Ib3 ( aacA4 ), which was considered a potential pseudogene in A. baumannii AB5075, to be functional providing broad resistance to gentamicin, kanamycin, amikacin, streptomycin and tobramycin when overexpressed in A. baumannii AB5075. Because p1AB5075 is transferrable to a wide range of environmental and clinical A. baumannii strains and non- baumannii Acinetobacter species, the relevance of our findings extends beyond A. baumannii AB5075.