Gtr9 mutation trades phage resistance for carbapenem sensitivity to potentiate phage-meropenem therapy against carbapenem-resistant Acinetobacter baumannii in vitro.

The combined use of phages and antibiotics offers an alternative avenue against multidrug-resistant bacteria. We have previously described the synergistic antibacterial effect of the phage pB23 and meropenem combination against carbapenem-resistant Acinetobacter baumannii (CRAB). The study uncovers the underlying molecular mechanism of phage resistance in CRAB mediated by a novel stop-gain mutation in the gene gtr9. Through phenotypic characterization of pleiotropy, including reduction of capsular polysaccharide production and biofilm formation caused by the mutation in gtr9, we revealed an evolutionary trade-off mechanism whereby phage-resistant CRAB exhibits reduced carbapenem resistance. The zebrafish infection model demonstrated that these phage-resistant mutants were attenuated in virulence in vivo. Throughout continuous passage experiments in vitro, gtr9 mutants displayed the stability of decreased growth rate, phage resistance, and virulence reduction. The combination therapy between phage pB23 and meropenem in different matrices exhibited consistent synergistic antibacterial activity in vitro, demonstrating its potential therapeutic in vivo. Collectively, our study reveals a trade-off mechanism underlying phage-antibiotic synergy, thereby providing a novel insight into bacterial resistance evolution and demonstrating the therapeutic potential of this approach against CRAB infections.