<i>Gtr9</i> mutation trades phage resistance for carbapenem sensitivity to potentiate phage-meropenem therapy against <i>carbapenem-resistant Acinetobacter baumannii in vitro</i>

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 <i>Acinetobacter baumannii</i> (CRAB). The study uncovers the underlying molecular mechanism of phage resistance in CRAB mediated by a novel stop-gain mutation in the gene <i>gtr9</i>. Through phenotypic characterization of pleiotropy, including reduction of capsular polysaccharide production and biofilm formation caused by the mutation in <i>gtr9</i>, 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 <i>in vivo</i>. Throughout continuous passage experiments <i>in vitro</i>, <i>gtr9</i> 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 <i>in vitro</i>, demonstrating its potential therapeutic <i>in vivo</i>. 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.