High-level polymyxin B resistance and underlying mechanism in a multidrug-resistant Acinetobacter strain isolated from the marine plastisphere.

Antibiotic resistance of Acinetobacter poses a serious threat to public health. Still, the antibiotic resistance features of marine Acinetobacter remain largely unexplored. Here, an Acinetobacter beijerinkii strain, named as MPE71, was isolated from the marine plastisphere. Phylogenetic analysis revealed close relationship of MPE71 with two human pathogenic strains of Acinetobacter. Minimum inhibitory concentration (MIC) assays of 10 antibiotics demonstrated the multidrug-resistance of MPE71. Notably, it showed a MIC of 200 µg/mL against polymyxin B. To the best of our knowledge, this is the first report demonstrating that Acinetobacter from the marine plastisphere exhibits multidrug resistance, including high-level resistance to polymyxin B. Genomic resistance gene analysis revealed numerous multidrug efflux pump genes encoded in MPE71. The resistance mechanism was further studied using transcriptomics. Enhanced membrane biosynthesis, multidrug efflux pumps, and periplasmic protein factors largely explained the resistance of MPE71 to polymyxin B, with up-regulation of relevant genes in a dose-dependent manner. Subsequently, inhibition of the proton motive force (PMF) by carbonyl cyanide 3-chlorophenylhydrazone diminished MPE71 resistance to polymyxin B, underscoring the key role of PMF-dependent efflux pumps. This study provides new evidence for the ecological risk of antibiotic resistance in the marine plastisphere.