Transient AdeABC-mediated eravacycline resistance in carbapenem-resistant Acinetobacter baumannii ST2: a mutation-independent efflux pump adaptation

PURPOSE: This study evaluated the impact of sub-MIC eravacycline exposure on carbapenem-resistant Acinetobacter baumannii with tigecycline MIC ≥ 8 µg/mL in clinical laboratory testing, and analyzed the resistance mechanisms. METHODS: 2144 A. baumannii isolates (2021–2024) from a Chinese tertiary hospital were analyzed for resistance trends. For 43 strains with tigecycline MIC ≥ 8 µg/mL, we conducted: (i) in vitro induction of resistance via serial sub-MIC eravacycline passage, (ii) efflux pump inhibition assays using phenylalanine-arginine β-naphthylamide, (iii) quantification of efflux pump gene expression (adeB, adeS), (iv) genomic analysis of resistance-associated mutations. RESULTS: Eravacycline exhibited MIC50/90 values that were 2-4-fold lower than those of tigecycline, demonstrating superior in vitro efficacy. Exposure to sub-MIC levels of eravacycline significantly increased resistance to eravacycline, tigecycline, minocycline, and polymyxin B, with PAβN reversing these effects. The expression of efflux pump-related genes adeB and adeS was upregulated, especially in eravacycline-induced strains. Although 88.37% of strains lacked mutations, eravacycline resistance demonstrated greater stability in adeB or adeS mutants. In 97.06% of strains with tigecycline MIC ≥ 4 µg/mL, the induced eravacycline MICs exceeded achievable alveolar concentrations. Eravacycline resistance demonstrated a correlation with tigecycline. And its combination with polymyxin B showed no synergistic effect. CONCLUSION: Eravacycline exhibited superior in vitro antibacterial efficacy against A. baumannii compared to tigecycline, but it is necessary to be alert to the resistance caused by the rapid increase in efflux pump expression. These findings highlight the need for cautious clinical use of eravacycline to mitigate resistance development.