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

The global spread of tigecycline resistance conferred by <i>tet</i>(X3) poses a serious threat to clinical treatment of multidrug-resistant (MDR) <i>Acinetobacter</i> infections. Despite <i>tet</i>(X3) being detected in diverse <i>Acinetobacter</i> species, its transposition mechanism and fitness in these pathogens remain poorly characterized. Here, we reported the first plasmid-borne IS<i>Aba52</i>-mediated transposable element harboring <i>tet</i>(X3) in <i>Acinetobacter amyesii</i> YH16040. Conjugation experiments demonstrated the transferability of <i>tet</i>(X3) into the chromosome of <i>Acinetobacter baylyi</i> ADP1 at an efficiency of (7.1 ± 2.5) × 10^-8. High-throughput sequencing revealed six tandem copies of IS<i>Aba52</i>-flanked <i>tet</i>(X3), <i>floR</i>, and <i>sul2</i> forming a 231.6 Kb complex transposon in the obtained transconjugant <i>A. baylyi</i> 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 IS<i>Aba52</i>-mediated transposition in disseminating <i>tet</i>(X3) between <i>Acinetobacter</i> species and elucidate the fitness changes employed by MDR strains under antibiotic selection pressures. Our study advocates the urgent need for surveillance of IS<i>Aba52</i>-associated resistance elements in human, animal, and environmental settings.