Transfer and Fitness of ISAba52-Mediated tet(X3) Transposon in Acinetobacter spp

The global spread of tigecycline resistance conferred by tet(X3) poses a serious threat to clinical treatment of multidrug-resistant (MDR) Acinetobacter infections. Despite tet(X3) being detected in diverse Acinetobacter species, its transposition mechanism and fitness in these pathogens remain poorly characterized. Here, we reported the first plasmid-borne ISAba52-mediated transposable element harboring tet(X3) in Acinetobacter amyesii YH16040. Conjugation experiments demonstrated the transferability of tet(X3) into the chromosome of Acinetobacter baylyi ADP1 at an efficiency of (7.1 ± 2.5) × 10^-8. High-throughput sequencing revealed six tandem copies of ISAba52-flanked tet(X3), floR, and sul2 forming a 231.6 Kb complex transposon in the obtained transconjugant A. baylyi YH16040C. Phenotypic assays showed that YH16040C exhibited elevated resistance to tigecycline, chlortetracycline, florfenicol, and trimethoprim-sulfamethoxazole by 64- to 256-fold. Notably, YH16040C exhibited a growth advantage, reduced competition ability, and non-significant difference in biofilm formation compared to ADP1 in antibiotic-free backgrounds. Under moderate antibiotic treatment of tigecycline, chlortetracycline, florfenicol, and trimethoprim-sulfamethoxazole, the competition ability of YH16040C against ADP1 was significantly higher than that without antibiotics. All of these highlight the importance of ISAba52-mediated transposition in disseminating tet(X3) between Acinetobacter species and elucidate the fitness changes employed by MDR strains under antibiotic selection pressures. Our study advocates the urgent need for surveillance of ISAba52-associated resistance elements in human, animal, and environmental settings.