From screening to treatment: in vitro and in vivo efficacy of phage-meropenem combination against highly virulent Carbapenem-resistant Acinetobacter baumannii

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a hospital-acquired pathogen of significant clinical importance. Given the slow progress in antibiotic development, phages have emerged as a novel therapeutic strategy against CRAB infections due to their synergistic effects with antibiotics. This study aims to explore phage-antibiotic combination therapy targeting CRAB, thereby providing a theoretical foundation for its clinical translation. Through virulence assessments in Galleria mellonella larvae and mice, CRAB110 was identified as the most virulent strain among 199 clinical CRAB isolates. Transcription analysis revealed that its enhanced pathogenicity may be associated with dysregulated pro-inflammatory responses. Using CRAB110 as the host bacterium, the newly isolated phage, P2, was selected for further study due to its high lytic efficiency and broad host range. The CRAB110 mutant strain CRAB110-2N, treated with P2, exhibited increased drug susceptibility and attenuated virulence compared to the wild-type strain. Both in vitro and in vivo experiments demonstrated that the combination of P2 with antibiotics achieved superior bacterial clearance compared to phage therapy alone. Furthermore, whole-genome analysis of CRAB110-2N revealed a mutation in its ugd . The constructed CRAB110Δ ugd showed tolerance to P2; the quorum-sensing gene abaI was associated with phage susceptibility, with the CRAB110Δ abaI strain displaying insensitivity to P2. These findings provide experimental evidence supporting the clinical application of phage-antibiotic combination therapy. IMPORTANCE This study focused on phage-antibiotic combination therapy targeting carbapenem-resistant Acinetobacter baumannii (CRAB), a priority pathogen designated by the World Health Organization (WHO). A highly virulent strain, CRAB110, was screened from 199 clinical isolates, and a novel, highly lytic phage, P2, was isolated from sewage samples. CRAB110-2N, a mutant strain induced by P2 exposure, displayed increased antibiotic susceptibility and attenuated virulence. Key findings showed that the combination of P2 and meropenem achieved a 99% bacterial clearance rate in vitro and exerted significant synergistic effects in vivo . Genomic analysis identified mutations in the ugd gene, and knockout experiments confirmed its correlation with phage susceptibility. Additionally, the quorum-sensing gene abaI was found to be associated with phage susceptibility in CRAB. Collectively, these results provide critical experimental evidence for the clinical translation of this phage-antibiotic combination therapy.